Add Owned<T> behaviour comparison suite vs Autofac (tests for #147)

[ekalchev]

Follows up on #147.

Adds an Owned<T> behaviour-comparison suite under tests/Pure.DI.UsageTests/Owned/ (14 files, 28 tests). It uses differential testing: each test asserts Pure.DI and Autofac's Owned<T> against the same expected value, with Autofac as a proven reference implementation. That gives two things at once:

• regression coverage for the scenarios that already behave correctly (23 tests), and
• executable, reproducible failing tests for the divergences reported in Owned<T>: Func<Owned<T>> and nested Owned<T> share one accumulator → disposal leaks & cross-contamination #147 (5 tests).

⚠️ 5 tests fail on purpose

They document the defects from #147 and are expected to stay red until Owned<T> is fixed:

• MessagePumpTests — a captured Func<Owned<T>> reuses one accumulator per resolution, so in the canonical message-pump loop only the first unit of work is disposed; the rest leak.
• NestedOwnedTests (×2) — outer and inner Owned<T> share that same accumulator, so disposing either disposes both graphs.
• DisposalExceptionTests — Owned<T>.Dispose() swallows exceptions thrown by a component's Dispose(); Autofac surfaces them.
• SingletonOwnerTests — a never-released Owned<T> is not disposed on composition disposal; Autofac disposes it as a container-level safety net.

The first three share one root cause (a single Owned accumulator reused per composition-root resolution instead of one per Owned<T>).

23 tests confirm parity

Transient/singleton/scoped disposal, root isolation, owned-of-singleton lifetime, mixed-lifetime nested graphs, diamond (shared) dependencies, LIFO disposal order, factory-created disposables, all async-disposal paths (including sync-over-async and mixed sync/async), double-dispose idempotency, and explicit release by a singleton owner.

No Autofac dependency by default

All Autofac code is compiled only under the AUTOFAC_REFERENCE symbol; the package reference is not committed (it lives in a git-ignored .csproj.user). Without the symbol — the default, and what CI sees — the suite builds and runs as pure Pure.DI tests, no Autofac package or reference. The same 5 tests fail either way; the Autofac side just adds an independent witness that the expected values are correct. See tests/Pure.DI.UsageTests/Owned/README.md for the enable recipe.

Verified locally on net10.0: builds with 0 warnings and runs 23 passed / 5 failed both with and without AUTOFAC_REFERENCE.

Run just this suite

dotnet test tests/Pure.DI.UsageTests/Pure.DI.UsageTests.csproj --filter "FullyQualifiedName~Owned."