chore: promote 0.9.0 API surface to PublicAPI.Shipped

[Chris-Wolfgang]

Release prep for 0.9.0

This promotes the accumulated unreleased public API surface from each project's PublicAPI.Unshipped.txt into its PublicAPI.Shipped.txt, reflecting that this API is now shipping in 0.9.0. Each PublicAPI.Unshipped.txt is reset to just the #nullable enable header.

What moved to Shipped

Wolfgang.Etl.TestKit (42 entries):

• Factory-based TestExtractor<T> constructors (feat: add factory-based constructors to TestExtractor<T> (#13) #171) — Func<T> / Func<int, T> with optional count and IProgressTimer
• Faulty* test doubles — FaultyExtractor<T>, FaultyLoader<T>, FaultyTransformer<T> with ThrowAt, ThrowAfterCompletion, DuplicateAt builders and overrides
• Dispose(bool) overrides on all six test doubles (fix: unsubscribe Elapsed on Dispose for all doubles #184)

Wolfgang.Etl.TestKit.Xunit (35 entries):

• ProgressCapture<T> and ProgressAssert helpers (feat: add ProgressCapture<T> and ProgressAssert helpers (#14) #172)
• Idempotent* contract base classes — IdempotentExtractorContractTests, IdempotentLoaderContractTests, IdempotentTransformerContractTests, including the twice-called reset tests and the GetCurrentItemCount virtual hooks (feat: add opt-in idempotency contract test base classes (#15) #176/test: add CurrentItemCount per-run reset tests to idempotency bases (#15) #178)

Verification

• dotnet build ETL-Test-Kit.slnx -c Release — 0 warnings / 0 errors across all TFMs (RS0016/RS0017 confirm the Shipped union is exact)
• Tests green: 122 (TestKit) + 236 (Xunit) = 358 passing on net8.0

After this merges, both PublicAPI.Unshipped.txt files contain only #nullable enable.

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