Complete the managed .NET interop analyzer boundary

[nickna]

Summary

• route compile-time external IL emission, declaration inspection, extension imports, external type synthesis, and arbitrary CLR attribute discovery through the guarded managed interop boundary
• preserve full BCL, embedder, and third-party assembly support in the Managed SKU
• keep the closed Obsolete, Serializable, and NonSerialized mappings rooted and usable by the Native compiler while rejecting arbitrary open-world attribute discovery there
• reduce the structured project analyzer baseline from 48 warnings to an empty inventory
• add a Native AOT smoke test for the rooted known-attribute contract and update the AOT plan and measurements

Validation

• AOT analyzer ratchet: 0 warnings, exact baseline match (down from 48)
• managed test suite: 16,262 passed
• focused interop suites: 203 passed
• decorator attribute suite after the closed-constructor correction: 6 passed
• Native win-arm64 smoke: known attribute output true; main output 42; feature output 2 2 33; arbitrary dynamic .NET interop rejected with the managed-build diagnostic
• Native image: 93,376,000 bytes, 18,432 bytes (0.0197%) smaller than merged main

The zero baseline covers SDK project analyzers. Full Native AOT publish can still emit separate ILCompiler and dependency diagnostics; the plan now calls out that distinction explicitly.