feat(nsubstitute): milestone 5 phase 2 - test suites and verification

[ncipollina]

📋 Summary

Implements PLAN-0005 Phase 2: test coverage for Compono.NSubstitute (Phase 0/1's engine extension point and provider package) and a real, packaged end-to-end verification. Writing this phase's own regression coverage caught two real bugs in already-merged Phase 1/Milestone-1 code, both fixed here with their own regression tests.

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📝 Changes

test/Compono.NSubstitute.Tests (new project, 23 tests × 2 TFMs)

• IsSubstitutableTests.cs — direct coverage of NSubstituteProvider.IsSubstitutable (interface / delegate / abstract-class with the option on and off / sealed class / struct / string), including the System.Delegate/System.MulticastDelegate negative cases ADR-0025 Amendment 1 calls out.
• NSubstituteProviderTests.cs — TryProvide's outcome exercised through a real Composer (CompositionProviderResult's Value/IsHandled are internal to Compono, so this is the only way to observe it from outside), plus the options-mutation snapshot regression test.
• CompositionBuilderExtensionsTests.cs — UseNSubstitute()/UseNSubstitute(configure) wiring a working provider into a real Composer.
• EndToEndCompositionTests.cs — the plan's own [Shared]-via-CompositionRow reuse shape, run against the real NSubstituteProvider.
• PublicApiSurfaceTests.cs — locks Compono.NSubstitute's exact public type set, matching Compono.XunitV3.Tests' existing pattern.

Real end-to-end verification (test/Compono.XunitV3.SampleTests)

• Added a Compono.NSubstitute package reference (packed from current source via pack-to-local-feed.sh, not a ProjectReference) and NSubstituteTests.cs, running the plan's own Goal-section scenario verbatim under a real xUnit v3 runner.

Two real bugs found and fixed while writing this phase's tests:

1. NSubstituteProvider.IsSubstitutable misclassified System.Delegate/System.MulticastDelegate themselves as substitutable abstract classes — ADR-0025 Amendment 1's stated intent was only half-enforced. Fixed by excluding typeof(Delegate).IsAssignableFrom(requestedType) from the abstract-class branch.
2. The real end-to-end run exposed a CMP0003 compile error on the plan's own Goal snippet ([Shared] IOrderRepository as a bare xUnit theory parameter): Compono.Generators' LeafTypeClassifier.IsRuntimeProviderResolved still hard-rejected an interface/abstract-class/delegate root, a stale pre-Milestone-5 assumption from before public providers existed. Loosened to match the member-level classification (ADR-0024 Amendment 2), with four rewritten and one new Compono.Generators.Tests snapshot test proving concrete-type roots are unaffected.

Docs

• docs/adr/0024-public-provider-extensibility-model.md — Amendment 2 records the CMP0003 root-check change and why it ships as an ordinary fix, not breaking.
• docs/mvp.md, docs/public-api.md — Milestone 5 / NSubstitute Integration now say "implemented and test-covered/end-to-end verified."
• docs/plans/0005-milestone-5-nsubstitute-integration.md — Phase 2 checked off and closed out with a detailed Notes entry.

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🧪 Validation

• Build/test status: whole-solution dotnet build/dotnet test — 720/720 passing (Compono.Tests 412, Compono.Generators.Tests 168, Compono.XunitV3.Tests 94, Compono.NSubstitute.Tests 46, all ×2 TFMs), 0 warnings.
• Manual verification performed: separate real dotnet test run of test/Compono.XunitV3.SampleTests against packages packed fresh from current source (not ProjectReference) — NSubstituteTests.Saves_order passes end-to-end.
• Edge cases checked: Delegate/MulticastDelegate framework base types vs. a real custom delegate type; abstract-class substitution with the option on/off; options-instance mutation after registration has no effect on an already-registered provider; a concrete sealed type with no accessible constructor still reports CMP0002 at a composed root (proving the CMP0003 loosening is scoped correctly).

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💬 Notes for Reviewers

The CMP0003 change (item 2 above) is the part most worth a close look — it's a generator behavior change, not test-only, made after confirming the direction with the repo owner rather than unilaterally. It's additive (previously non-compiling code now compiles; nothing that compiled before changes meaning), so no breaking label applied, per ADR-0024's own Alpha Compatibility Policy.

PLAN-0005 Phase 3 (docs/cleanup) remains open as its own follow-up PR.