feat(shims): suppress SHIM001 for consumer-bridged types (#69 follow-up)

[ChrisonSimtian]

Summary

Cleans up the 20 SHIM001 warnings left over from session 4 (the 10 CI host singleton hand-bridges × 2 referenced-assembly paths).

The TransitionShimGenerator now pre-walks the consuming compilation's own namespaces under the target shim prefix and collects FQNs of every top-level public type the consumer has hand-written. When the canonical-side walk would emit a shim for a type whose target FQN already exists in that set, the generator silently defers to the hand-bridge — no source emitted, no SHIM001 diagnostic.

Why no annotation?

Considered an explicit [assembly: SkipShimForCanonical(typeof(...))] marker. Rejected because:

• The generator already auto-discovers everything from the canonical assembly walk; symmetry says consumer-side hand-writes should be auto-discovered too.
• The hand-bridge file itself is the source of truth — adding a parallel attribute would risk drift.
• Implicit detection works for the CI hosts (session 4) AND the existing NukeBuild hand-bridge in Nuke.Common.NukeBuild.cs and any future hand-bridges with zero ceremony.

A canonical name + arity match via INamedTypeSymbol.MetadataName keeps arity-correctness for generics.

Live impact

Before this PR (the Nuke.Common shim build):

Kind	Count
enum	76
no-accessible-ctor	28
delegate	14
struct	4
ambiguous-across-assemblies	4
sealed-class	2

After:

Kind	Count	Change
enum	76	—
no-accessible-ctor	8	−20 (the 10 CI hosts × 2 paths)
delegate	14	—
struct	4	—
ambiguous-across-assemblies	4	—
sealed-class	2	—

The 4 remaining unique no-accessible-ctor types — AbsolutePath, AzureKeyVault, MSBuildProject, DelegateDisposable — are the natural session-5 candidates.

Verification

• New generator test SkipsCanonicalTypesAlreadyHandBridgedByConsumer covers both the sealed-class case (would otherwise warn) and the regular-class case (would otherwise emit). Both should be silent when a hand-bridge exists.
• 6/6 Fallout.SourceGenerators.Tests pass — no snapshot regression on the existing 5.
• Nuke.Common shim builds clean (0 errors) with the 20 CI host SHIM001 warnings now silent.
• tests/Nuke.Common.Shim.Tests still builds clean.

Test plan

• CI green
• Live shim build: confirm the 10 CI host SHIM001 warnings are gone
• Existing NukeBuild hand-bridge is also auto-discovered (no behavior change there since the canonical name is FalloutBuild, not NukeBuild — different name, no collision)

🤖 Generated with Claude Code