refactor(core): extract the product-agnostic foundation into Nordstein.Core - #535

A spike of the first slice of shared-code extraction, ahead of a second Nordstein product. It moves the least entangled foundation code into core/ as Nordstein.Core and validates the full mechanism — packaging, namespaces, dependency direction, Docker restore layers, CI, and the extraction itself — before a second repository exists to get any of it wrong.

Nothing is published. The feed and the licence are open decisions, recorded in core/PUBLISHING.md.

What moved

Core is its own solution and deliberately not part of Proxytrace.sln. The moment it only compiles as part of the product, it stops being extractable and nobody finds out until the split is attempted.

How the product consumes it

Consuming projects declare a NordsteinCoreReference item rather than a reference — see Proxytrace.Domain.csproj for a live example, and Directory.Build.targets for the expansion.

Directory.Build.targets turns that item into a ProjectReference (source mode — the default while Core's sources are present) or a PackageReference at $(NordsteinCoreVersion) (package mode).

The indirection is the load-bearing part. Without it, every cross-boundary change becomes edit → pack → bump → restore → retest, and the predictable result is that nobody makes small Core improvements any more — they copy the code into the product instead, which is the exact failure the extraction exists to prevent. Once Core is its own repository, NordsteinCorePath points at a sibling checkout (../Core/) and nothing else changes.

What keeps the boundary honest

Staging Core inside this repository is convenient but removes the natural barrier — nothing but review stops a Proxytrace type being referenced in there. Two mechanical guards replace it:

• backend builds and tests core/Nordstein.Core.sln standalone, before the product.
• core-package (new job) packs Core and rebuilds the whole product against the resulting .nupkg files. Project references hide two consumer-only failures: a type that is public in source but never made it into the package surface, and a dependency Core forgot to declare because source mode resolved it through the product's own graph. Packages are uploaded as an artifact, not pushed.

The extraction was dry-run, and it did not work the first time

The point of a spike is to find this now rather than on the day of the split. Splitting core/ into a scratch worktree turned up two defects, both fixed in 0efa3b0:

1. The extracted repository would not have built at all. core/Directory.Build.props imported the parent Directory.Build.props unconditionally. Standalone there is no file above it, GetPathOfFileAbove returns empty, and importing an empty project path fails with MSB4020 before a single project compiles. The import is now conditional, and the three things the parent supplies that Core needs are defaulted locally.

2. git subtree split is the wrong tool and loses the history. It filters strictly by path and does not follow renames — and everything under core/ was git mv'd from Proxytrace.Common/ and friends. Measured: 1 commit. git filter-repo with the pre-move paths mapped onto the current ones keeps the real history: 7 commits, back through Trace sessions: higher-level live grouping for debugging #371, fix(storage,di): invalidate the entity cache after commit; populate framework plumbing once (#450, #451) #465, Various Bugfixes #484, Bump the nuget-minor-patch group with 9 updates #508 and the background-service hosting fix. core/PUBLISHING.md now carries the verified recipe, the ordering constraint on the renames, and the standalone build/test/pack check to run before pushing the new repository.

An earlier revision of this description claimed a subtree split would preserve the past. That was wrong; this is the correction.

Other changes worth a reviewer's attention

• Dockerfiles (Proxytrace.Api, Proxytrace.Proxy.Api, deploy/allinone) copy Directory.Build.props/.targets, nuget.config and Core's project file into the restore layer. Without them the reference expansion yields nothing and restore silently misses the dependency.
• nuget.config is new: a clear element plus nuget.org, so a machine-level source cannot supply a package this repository did not intend to restore. The local package-mode feed comes from Directory.Build.props by absolute path — NuGet resolves a relative RestoreAdditionalProjectSources per project directory, which fails only once the packages are absent from the global cache (that is how it passed locally and failed in CI; see the thread).
• detect-changes treats ^core/, Directory.Build.targets and nuget.config as backend changes.
• Core's tests are not in dotnet test Proxytrace.sln. A cross-cutting local run now needs both solutions; docs/testing.md and docs/commands.md say so.

Docs

New docs/code-reuse.md (indexed in CLAUDE.md) covers the mechanism, the one rule — Core may not reference the product — and the next slices with what blocks each: the domain/storage foundation (assembly-hardwired reflection in Domain.Module/Storage.Module, internal seams, MigrationsAssembly pinning), then licensing, then the cross-cutting subsystems, then the frontend primitives.

architecture.md, ci.md, commands.md, testing.md, security.md, validation.md, code-style.md, releasing.md and the create-domain / test skills follow the rename.

No user-facing change, so no CHANGELOG entry.

Verification

All 16 CI checks are green on 0efa3b0, including core-package (compiles the product against the packed .nupkg files) and backend (both solutions, with PROXYTRACE_REQUIRE_DOCKER_TESTS=true), plus e2e and the all-in-one image boot.

Locally, all without -p:NuGetAudit=false since #534 landed:

Full suite rather than a narrow scope, because the change is cross-cutting by definition.

Open decisions (not in this PR)

1. Feed. Proxytrace is public and Elastic-licensed, so a private feed would make it unbuildable outside the organisation (GitHub Packages has no anonymous read). Private source with public packages keeps both properties. If Proxytrace is going closed anyway, that constraint disappears.
2. Licence. The packed core/LICENSE is a placeholder copy of the product's. A licence cannot be recalled from consumers who already restored the package.
3. Prefix reservation for Nordstein.* on nuget.org, before any public push.

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