refactor(core): extract the product-agnostic foundation into Nordstein.Core

.claude/skills/create-domain/SKILL.md

@@ -134,7 +134,7 @@ internal record TestRunGroup : DomainEntity<ITestRunGroup>, ITestRunGroup
 }
 ```
 
-### Validation helpers (from `Proxytrace.Common.Validation`)
+### Validation helpers (from `Nordstein.Core.Common.Validation`)
 
 ```csharp
 yield return Validation.NotNullOrWhiteSpace(Name);  // note capital S

.claude/skills/test/SKILL.md

@@ -43,7 +43,7 @@ substitute in the container instead (see *Injecting fakes* below).
 | `Proxytrace.Application.Tests` | Application services end-to-end (e.g. `TestRunnerService`) with faked infrastructure | `BaseTest<Module>` |
 | `Proxytrace.Api.Tests` | HTTP controllers / routing | `BaseTest<Module>` |
 | `Proxytrace.Infrastructure.Tests` | `ModelClient` and external integration wrappers | `BaseTest<Module>` |
-| `Proxytrace.Serialization.Tests`, `Proxytrace.Common.Tests`, `Proxytrace.Proxy.Tests`, `Proxytrace.Messaging.Tests`, `Proxytrace.Licensing.Tests` | Their respective layers | `BaseTest<Module>` |
+| `Proxytrace.Serialization.Tests`, `Nordstein.Core.Common.Tests`, `Proxytrace.Proxy.Tests`, `Proxytrace.Messaging.Tests`, `Proxytrace.Licensing.Tests` | Their respective layers | `BaseTest<Module>` |
 
 Each test project ships **one `Module : Autofac.Module`** that wires the layer under test
 plus in-memory storage and the standard infrastructure stubs. This per-project module *is*
@@ -53,7 +53,7 @@ the shared baseline — there are no other shared fixtures. See *The per-project
 
 ## The base test harness
 
-All tests extend `BaseTest<TModule>` from `Proxytrace.Testing`:
+All tests extend `BaseTest<TModule>` from `Nordstein.Core.Testing`:
 
 ```csharp
 [TestClass]
@@ -84,7 +84,7 @@ public sealed class MyTests : BaseTest<Module>   // or DomainTest<Module>
 ### How the harness works (and why it's stateless)
 
 - `GetServices(action)` builds a **brand-new Autofac container every call**: it registers
-  `Proxytrace.Testing.Module`, then your `TModule`, then runs `ConfigureContainer`, then
+  `Nordstein.Core.Testing.Module`, then your `TModule`, then runs `ConfigureContainer`, then
   your per-call `action`. Each container has its **own isolated in-memory database**.
 - The container is recorded in `TestContext.Properties["Containers"]` and disposed in
   `[TestCleanup]`. You never manage container lifetime yourself.

.github/actions/detect-changes/action.yml

@@ -66,7 +66,9 @@ runs:
           all=true
         fi
 
-        backend_re='^Proxytrace\.|^Proxytrace\.sln$|^Directory\.Build\.props$|^coverage\.runsettings$|^dotnet-tools\.json$'
+        # `^core/` is the Nordstein.Core staging area: the product consumes it as a project or
+        # package reference, so a change there is a backend change like any other.
+        backend_re='^Proxytrace\.|^Proxytrace\.sln$|^core/|^Directory\.Build\.props$|^Directory\.Build\.targets$|^nuget\.config$|^coverage\.runsettings$|^dotnet-tools\.json$'
         frontend_re='^frontend/'
         # The all-in-one image bundles both halves of the app plus its own packaging.
         # Deliberately NOT including `^manual/`: the image build does compile the manual,

.github/workflows/ci.yml

@@ -166,21 +166,80 @@ jobs:
           cache-dependency-path: |
             **/*.csproj
 
+      # Two solutions: Nordstein.Core is a separate, product-agnostic solution that Proxytrace
+      # consumes (see docs/code-reuse.md). It is built and tested first because the product
+      # depends on it, and separately because it must keep standing on its own — the moment it
+      # only compiles as part of Proxytrace.sln, it is no longer extractable.
       - name: Restore
-        run: dotnet restore Proxytrace.sln --nologo
+        run: |
+          dotnet restore core/Nordstein.Core.sln --nologo
+          dotnet restore Proxytrace.sln --nologo
 
       - name: Build
-        run: dotnet build Proxytrace.sln --nologo --no-restore
+        run: |
+          dotnet build core/Nordstein.Core.sln --nologo --no-restore
+          dotnet build Proxytrace.sln --nologo --no-restore
 
       # PROXYTRACE_REQUIRE_DOCKER_TESTS turns the container-backed tests (currently the real-Redis
       # ingestion transport ones) from skip-if-unavailable into hard failures. They must stay
       # skippable locally so `dotnet test` never requires Docker, but a runner that quietly lost its
       # container runtime would then drop exactly the coverage those tests exist to guarantee.
       - name: Test
-        run: dotnet test Proxytrace.sln --nologo --verbosity minimal --no-restore
+        run: |
+          dotnet test core/Nordstein.Core.sln --nologo --verbosity minimal --no-restore
+          dotnet test Proxytrace.sln --nologo --verbosity minimal --no-restore
         env:
           PROXYTRACE_REQUIRE_DOCKER_TESTS: true
 
+  # Packs Nordstein.Core and rebuilds the product against the resulting .nupkg files instead of
+  # the sources. Everyday builds use the project references, which hide two things a consumer
+  # would hit: a type that is public in source but never made it into the package's surface, and
+  # a dependency Core forgot to declare (source mode resolves it through the product's own
+  # graph). Catching that here is the difference between finding it now and finding it in the
+  # first repository that has no Proxytrace sources to fall back on.
+  core-package:
+    needs: changes
+    if: ${{ inputs.full || needs.changes.outputs.backend == 'true' }}
+    runs-on: ubuntu-latest
+    timeout-minutes: 20
+
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v7
+
+      - name: Setup .NET 10 SDK
+        uses: actions/setup-dotnet@v6
+        with:
+          dotnet-version: 10.0.x
+          cache: true
+          cache-dependency-path: |
+            **/*.csproj
+
+      - name: Pack Nordstein.Core
+        run: >-
+          dotnet pack core/Nordstein.Core.sln --nologo -c Release
+          -p:NordsteinCoreVersion=0.1.0-ci.${{ github.run_number }}
+          -o core/artifacts
+
+      # The local feed is picked up from core/artifacts by Directory.Build.props; it is not
+      # passed here, because a relative source path resolves per project directory and silently
+      # only fails once the packages are absent from the global cache.
+      - name: Build the product against the packages
+        run: >-
+          dotnet build Proxytrace.sln --nologo
+          -p:UseLocalCore=false
+          -p:NordsteinCoreVersion=0.1.0-ci.${{ github.run_number }}
+
+      - name: Upload packages
+        uses: actions/upload-artifact@v4
+        with:
+          name: nordstein-core-packages
+          path: |
+            core/artifacts/*.nupkg
+            core/artifacts/*.snupkg
+          if-no-files-found: error
+          retention-days: 7
+
   # Builds the released all-in-one image and boots it, so a container that fails to come up
   # (initdb, supervisord, nginx upstream, migrations on a cold volume) is caught here rather
   # than after a release tag is pushed. Single-arch: the release build adds arm64.

.gitignore

@@ -66,3 +66,6 @@ perf/results/
 
 # Prompt-lab transcripts (generated by the prompt-lab skill; real model calls, not build output)
 .prompt-lab/
+
+# Locally packed Nordstein.Core packages (see nuget.config)
+core/artifacts/

CLAUDE.md

@@ -7,6 +7,7 @@ Detailed guidance lives in [`docs/`](docs/). Read the relevant page **before** w
 | Doc | Read before… |
 |-----|--------------|
 | [`docs/architecture.md`](docs/architecture.md) | Touching project structure, layering, or Autofac DI/modules |
+| [`docs/code-reuse.md`](docs/code-reuse.md) | Touching anything under `core/` (Nordstein.Core), or deciding whether new code is product-agnostic |
 | [`docs/code-style.md`](docs/code-style.md) | Writing any backend C# — style rules + key conventions |
 | [`docs/domain-entities.md`](docs/domain-entities.md) | Adding/changing a domain entity (the five-file pattern, FK conventions, factory delegates) |
 | [`docs/validation.md`](docs/validation.md) | Adding domain validation rules |
@@ -61,7 +62,7 @@ Detailed guidance lives in [`docs/`](docs/). Read the relevant page **before** w
   - **e2e / perf** — never as a routine check. Run them only when the change is in that flow or the
     user asks; both boot Docker stacks and take many minutes.
   - **Run the full suite** (`dotnet test Proxytrace.sln`) only when the change is genuinely
-    cross-cutting — `Proxytrace.Common`, `Proxytrace.Testing`, DI/module wiring, a shared interface
+    cross-cutting — `Nordstein.Core.Common`, `Nordstein.Core.Testing`, DI/module wiring, a shared interface
     signature, a package bump — or when cutting a release. Say which scope you ran and why, so a
     narrow run is never mistaken for a full one.
 - **Internationalization** — the UI is multilingual (English is the source). Every user-facing
@@ -75,7 +76,7 @@ Detailed guidance lives in [`docs/`](docs/). Read the relevant page **before** w
   dedup, title/body quality, and labels — then carry on with your task.
 - **Nullable suppression** — suppressing nullable warnings with `!` is strictly forbidden everywhere.
   There is exactly **one** sanctioned exception, and it is not extensible: `Validation.Success` in
-  [`Proxytrace.Common/Validation/Validation.cs`](Proxytrace.Common/Validation/Validation.cs). The BCL
+  [`core/Nordstein.Core.Common/Validation/Validation.cs`](core/Nordstein.Core.Common/Validation/Validation.cs). The BCL
   defines validation success as a `null` `ValidationResult` while declaring
   `IValidatableObject.Validate` to return a **non-nullable** element type, so the framework demands a
   value it defines as null through a signature we cannot change. That single line is documented in

Directory.Build.props

@@ -8,4 +8,51 @@
          displayed verbatim in the UI and sent to the license server. -->
     <IncludeSourceRevisionInInformationalVersion>false</IncludeSourceRevisionInInformationalVersion>
   </PropertyGroup>
+
+  <!-- ── Nordstein.Core consumption ────────────────────────────────────────────────
+       The reusable foundation (Nordstein.Core.Common, Nordstein.Core.Testing) is
+       product-agnostic code that is destined for its own repository and its own NuGet
+       packages. It currently lives under core/ in this repository as a staging area, so
+       the split can be completed with a `git subtree split` on the core/ prefix without
+       losing history.
+
+       A project consumes it by declaring an item rather than a reference:
+
+           <ItemGroup>
+             <NordsteinCoreReference Include="Nordstein.Core.Common" />
+           </ItemGroup>
+
+       Directory.Build.targets turns that into either a ProjectReference (source mode) or a
+       PackageReference (package mode). Source mode is what you want while developing: F12
+       navigates into Core, one build covers both halves, and a Core edit is picked up
+       without a pack/restore round trip. Without it, a cross-repository change becomes
+       "edit, pack, bump, restore, retest" and nobody makes small Core improvements any more.
+
+       Mode is chosen automatically by whether the Core sources are present, and can be
+       forced either way:
+
+           dotnet build -p:UseLocalCore=false -p:NordsteinCoreVersion=0.1.0-dev
+
+       Once Core moves out, point NordsteinCorePath at the sibling checkout
+       (../Core/) — nothing else changes. -->
+  <PropertyGroup>
+    <NordsteinCorePath Condition="'$(NordsteinCorePath)' == ''">$(MSBuildThisFileDirectory)core/</NordsteinCorePath>
+    <UseLocalCore Condition="'$(UseLocalCore)' == '' AND Exists('$(NordsteinCorePath)Nordstein.Core.sln')">true</UseLocalCore>
+    <UseLocalCore Condition="'$(UseLocalCore)' == ''">false</UseLocalCore>
+    <!-- Only consulted in package mode. Kept in one place so a Core bump is a one-line diff. -->
+    <NordsteinCoreVersion Condition="'$(NordsteinCoreVersion)' == ''">0.1.0-dev</NordsteinCoreVersion>
+  </PropertyGroup>
+
+  <!-- Package mode before Core is published anywhere restores from a locally packed feed. The
+       path is built from $(MSBuildThisFileDirectory) so it is absolute: NuGet resolves a
+       relative RestoreAdditionalProjectSources against each *project* directory, not the
+       repository root, so passing `core/artifacts` on the command line asks for
+       `Proxytrace.Domain/core/artifacts` and fails with NU1301 — and only once the packages are
+       missing from the global cache, which makes it look like it works right up until CI.
+
+       Gated on the directory existing so a real package-mode build against a published feed is
+       unaffected. -->
+  <PropertyGroup Condition="'$(UseLocalCore)' != 'true' AND Exists('$(MSBuildThisFileDirectory)core/artifacts')">
+    <RestoreAdditionalProjectSources>$(RestoreAdditionalProjectSources);$(MSBuildThisFileDirectory)core/artifacts</RestoreAdditionalProjectSources>
+  </PropertyGroup>
 </Project>

Directory.Build.targets

@@ -0,0 +1,28 @@
+<Project>
+  <!-- Expands every <NordsteinCoreReference Include="Nordstein.Core.X" /> into the reference
+       kind the current mode calls for. See the comment in Directory.Build.props for why the
+       indirection exists.
+
+       This lives in Directory.Build.targets rather than .props because the item is declared in
+       the project body, which MSBuild evaluates after .props and before .targets.
+
+       Core's own projects reference each other with plain ProjectReferences and never declare
+       the item, so this is a no-op inside core/. -->
+
+  <ItemGroup Condition="'$(UseLocalCore)' == 'true'">
+    <ProjectReference Include="@(NordsteinCoreReference->'$(NordsteinCorePath)%(Identity)/%(Identity).csproj')" />
+  </ItemGroup>
+
+  <ItemGroup Condition="'$(UseLocalCore)' != 'true'">
+    <PackageReference Include="@(NordsteinCoreReference)" Version="$(NordsteinCoreVersion)" />
+  </ItemGroup>
+
+  <!-- Package mode against a version that was never restored fails deep inside NuGet with a
+       message that does not mention Core at all. Fail here instead, where the fix is obvious. -->
+  <Target Name="ValidateNordsteinCoreMode"
+          BeforeTargets="CollectPackageReferences"
+          Condition="'$(UseLocalCore)' != 'true' AND '@(NordsteinCoreReference)' != ''">
+    <Error Condition="'$(NordsteinCoreVersion)' == ''"
+           Text="NordsteinCoreVersion must be set when building in package mode (UseLocalCore=false)." />
+  </Target>
+</Project>

Proxytrace.Api.Tests/AgentCallsControllerHistogramTests.cs

@@ -14,7 +14,7 @@
 using Proxytrace.Domain.AgentCall;
 using Proxytrace.Domain.Completion;
 using Proxytrace.Domain.Session;
-using Proxytrace.Testing;
+using Nordstein.Core.Testing;
 
 namespace Proxytrace.Api.Tests;
 

Proxytrace.Api.Tests/AgentCallsControllerProposalsTests.cs

@@ -18,7 +18,7 @@
 using Proxytrace.Domain.AuditLog;
 using Proxytrace.Domain.TestSuite;
 using Proxytrace.Licensing;
-using Proxytrace.Testing;
+using Nordstein.Core.Testing;
 
 namespace Proxytrace.Api.Tests;
 

Proxytrace.Api.Tests/AgentCallsControllerSummaryTests.cs

@@ -13,7 +13,7 @@
 using Proxytrace.Domain.AuditLog;
 using Proxytrace.Domain.Completion;
 using Proxytrace.Domain.Session;
-using Proxytrace.Testing;
+using Nordstein.Core.Testing;
 
 namespace Proxytrace.Api.Tests;
 

Proxytrace.Api.Tests/AgentCallsControllerTests.cs

@@ -16,7 +16,7 @@
 using Proxytrace.Domain.Completion;
 using Proxytrace.Domain.Message;
 using Proxytrace.Domain.Usage;
-using Proxytrace.Testing;
+using Nordstein.Core.Testing;
 
 namespace Proxytrace.Api.Tests;
 

Proxytrace.Api.Tests/AgentsControllerTests.cs

@@ -10,7 +10,7 @@
 using Proxytrace.Domain.AgentCall;
 using Proxytrace.Domain.ModelEndpoint;
 using Proxytrace.Domain.Project;
-using Proxytrace.Testing;
+using Nordstein.Core.Testing;
 
 namespace Proxytrace.Api.Tests;
 

Proxytrace.Api.Tests/AnomaliesControllerTests.cs

@@ -11,7 +11,7 @@
 using Proxytrace.Domain.CustomAnomaly;
 using Proxytrace.Domain.Message;
 using Proxytrace.Domain.Usage;
-using Proxytrace.Testing;
+using Nordstein.Core.Testing;
 
 namespace Proxytrace.Api.Tests;
 

Proxytrace.Api.Tests/AuditLogControllerTests.cs

@@ -10,7 +10,7 @@
 using Proxytrace.Domain.AuditLog;
 using Proxytrace.Domain.Project;
 using Proxytrace.Domain.User;
-using Proxytrace.Testing;
+using Nordstein.Core.Testing;
 
 namespace Proxytrace.Api.Tests;
 

Proxytrace.Api.Tests/Auth/ApiKeyAuthenticationHandlerTests.cs

@@ -9,13 +9,13 @@
 using Proxytrace.Api.Auth;
 using Proxytrace.Api.Auth.Mcp;
 using Proxytrace.Api.Auth.Rest;
-using Proxytrace.Common.Security;
+using Nordstein.Core.Common.Security;
 using Proxytrace.Domain;
 using Proxytrace.Domain.ApiKey;
 using Proxytrace.Domain.ModelProvider;
 using Proxytrace.Domain.Project;
 using Proxytrace.Domain.User;
-using Proxytrace.Testing;
+using Nordstein.Core.Testing;
 
 namespace Proxytrace.Api.Tests.Auth;
 

Proxytrace.Api.Tests/Auth/AuthUserResolverTests.cs

@@ -9,7 +9,7 @@
 using Proxytrace.Application.Auth;
 using Proxytrace.Domain;
 using Proxytrace.Domain.User;
-using Proxytrace.Testing;
+using Nordstein.Core.Testing;
 
 namespace Proxytrace.Api.Tests.Auth;
 

Proxytrace.Api.Tests/Auth/SigningKeyProviderTests.cs

@@ -3,7 +3,7 @@
 using Microsoft.Extensions.DependencyInjection;
 using Microsoft.Extensions.Hosting;
 using Proxytrace.Api.Auth;
-using Proxytrace.Testing;
+using Nordstein.Core.Testing;
 
 namespace Proxytrace.Api.Tests.Auth;
 

Proxytrace.Api.Tests/AuthControllerTests.cs

@@ -18,7 +18,7 @@
 using Proxytrace.Domain.Notification;
 using Proxytrace.Domain.User;
 using Proxytrace.Licensing;
-using Proxytrace.Testing;
+using Nordstein.Core.Testing;
 
 namespace Proxytrace.Api.Tests;
 

Proxytrace.Api.Tests/Config/ConfigControllerTests.cs

@@ -2,7 +2,7 @@
 using NSubstitute;
 using Proxytrace.Api.Controllers;
 using Proxytrace.Domain.Kiosk;
-using Proxytrace.Common.Hosting;
+using Nordstein.Core.Common.Hosting;
 
 namespace Proxytrace.Api.Tests.Config;
 

Proxytrace.Api.Tests/CostLimitsControllerTests.cs

@@ -20,7 +20,7 @@
 using Proxytrace.Domain.Project;
 using Proxytrace.Domain.User;
 using Proxytrace.Licensing;
-using Proxytrace.Testing;
+using Nordstein.Core.Testing;
 
 namespace Proxytrace.Api.Tests;
 

Proxytrace.Api.Tests/CustomAnomalyDetectorsControllerTests.cs

@@ -13,7 +13,7 @@
 using Proxytrace.Domain.ModelEndpoint;
 using Proxytrace.Domain.Project;
 using Proxytrace.Licensing;
-using Proxytrace.Testing;
+using Nordstein.Core.Testing;
 
 namespace Proxytrace.Api.Tests;
 

Proxytrace.Api.Tests/EmailSettingsControllerTests.cs

@@ -12,7 +12,7 @@
 using Proxytrace.Application.Notifications;
 using Proxytrace.Domain.Notification;
 using Proxytrace.Domain.User;
-using Proxytrace.Testing;
+using Nordstein.Core.Testing;
 
 namespace Proxytrace.Api.Tests;
 

Proxytrace.Api.Tests/EvaluatorTestBenchControllerTests.cs

@@ -17,7 +17,7 @@
 using Proxytrace.Domain.TestCase;
 using Proxytrace.Domain.TestResult;
 using Proxytrace.Domain.TestSuite;
-using Proxytrace.Testing;
+using Nordstein.Core.Testing;
 
 namespace Proxytrace.Api.Tests;