refactor(core): consume Nordstein.Core as a git submodule

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

@@ -66,9 +66,10 @@ runs:
           all=true
         fi
 
-        # `^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$|^dotnet-tools\.json$'
+        # `^core(/|$)` and `.gitmodules`: Nordstein.Core is a git submodule at core/. A pointer
+        # bump shows in the diff as the bare path `core` (no trailing slash), and the product
+        # compiles against whatever it points at, so it is a backend change like any other.
+        backend_re='^Proxytrace\.|^Proxytrace\.sln$|^core(/|$)|^\.gitmodules$|^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

@@ -157,6 +157,8 @@ jobs:
     steps:
       - name: Checkout
         uses: actions/checkout@v7
+        with:
+          submodules: recursive
 
       - name: Setup .NET 10 SDK
         uses: actions/setup-dotnet@v6
@@ -206,6 +208,8 @@ jobs:
     steps:
       - name: Checkout
         uses: actions/checkout@v7
+        with:
+          submodules: recursive
 
       - name: Setup .NET 10 SDK
         uses: actions/setup-dotnet@v6
@@ -253,6 +257,8 @@ jobs:
     steps:
       - name: Checkout
         uses: actions/checkout@v7
+        with:
+          submodules: recursive
 
       - name: Set up Docker Buildx
         uses: docker/setup-buildx-action@v4

.github/workflows/codeql.yml

@@ -78,6 +78,10 @@ jobs:
     steps:
       - name: Checkout
         uses: actions/checkout@v7
+        with:
+          # C# is analysed source-only (build-mode: none); the submodule carries Core's .cs
+          # files, so without it Core would silently drop out of Proxytrace's scan.
+          submodules: recursive
 
       - name: Initialize CodeQL
         uses: github/codeql-action/init@v4

.github/workflows/e2e.yml

@@ -44,6 +44,8 @@ jobs:
     steps:
       - name: Checkout
         uses: actions/checkout@v7
+        with:
+          submodules: recursive
 
       - name: Set up Docker Buildx
         uses: docker/setup-buildx-action@v4

.github/workflows/perf.yml

@@ -36,6 +36,8 @@ jobs:
     steps:
       - name: Checkout
         uses: actions/checkout@v7
+        with:
+          submodules: recursive
 
       - name: Setup .NET 10 SDK
         uses: actions/setup-dotnet@v6

.github/workflows/release.yml

@@ -70,6 +70,8 @@ jobs:
     steps:
       - name: Checkout
         uses: actions/checkout@v7
+        with:
+          submodules: recursive
 
       # QEMU lets the amd64 runner emulate arm64 for the multi-arch build below.
       # The heavy compile steps cross-compile natively (the Dockerfile pins its build

.gitmodules

@@ -0,0 +1,3 @@
+[submodule "core"]
+	path = core
+	url = https://github.com/NordsteinSoftware/Nordstein.Core.git

Directory.Build.props

@@ -11,10 +11,10 @@
 
   <!-- ── 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.
+       product-agnostic code that has its own repository and, later, its own NuGet
+       packages. It lives under core/ as a git submodule of
+       github.com/NordsteinSoftware/Nordstein.Core: a Core change is committed there and
+       consumed here by bumping the submodule pointer.
 
        A project consumes it by declaring an item rather than a reference:
 
@@ -33,8 +33,8 @@
 
            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. -->
+       NordsteinCorePath defaults to the core/ submodule mount; point it elsewhere with
+       -p:NordsteinCorePath=../Core/ to build against a Core checkout kept outside the tree. -->
   <PropertyGroup>
     <NordsteinCorePath Condition="'$(NordsteinCorePath)' == ''">$(MSBuildThisFileDirectory)core/</NordsteinCorePath>
     <UseLocalCore Condition="'$(UseLocalCore)' == '' AND Exists('$(NordsteinCorePath)Nordstein.Core.sln')">true</UseLocalCore>