MS0024 trigger surface: recognise custom [Queryable] extension methods outside IrLinqMapping

[danfma]

Background

#205 wired MS0024 into the Phase B walker, but the diagnostic has no production trigger in the current build because:

• TryCaptureQueryableMeta is gated by IrLinqMapping.TryResolve, which recognises only System.Linq.Enumerable and System.Linq.Queryable.
• System.Linq.Queryable stages always have an IQueryable<T> receiver, which Phase B: emit MS0024 hard error when opt-in queryable lambda body falls outside MVP subset #205 classifies as implicit — silent bail.
• System.Linq.Enumerable stages take Func<…>, not Expression<Func<…>>, and carry no [Queryable] attribute.

Net effect: the explicit branch of IsExplicitQueryableOptIn cannot fire today, so user-authored unsupported bodies still leak through silently when they go through a custom [Queryable] extension method.

Proposal

Broaden trigger detection so MS0024 covers the real opt-in path the [Queryable] attribute promises:

1. Teach IrLinqMapping.TryResolve (or a sibling resolver) to recognise user-authored methods carrying [Queryable] — at least for the LINQ-pipe shape source.MyStage(lambda) where the lambda is the principal predicate / selector.
2. Or: hoist trigger detection out of the LINQ-chain walker entirely, so any call site with a lambda argument whose target parameter has [Queryable] or Expression<Func<…>> runs through the walker.

Either path turns MS0024 into a real, user-visible diagnostic.

Scope

• Pick the trigger-surface broadening strategy.
• Wire it into TryCaptureQueryableMeta (or a new analogous entry point).
• Add tests that cover a user-authored [Queryable]-tagged extension method with an unsupported body — expecting MS0024.
• Update IrExpressionTreeExtractor's class-level docstring to drop the "no production trigger" caveat once the surface is broadened.

References

• Phase B: emit MS0024 hard error when opt-in queryable lambda body falls outside MVP subset #205 — initial MS0024 plumbing.
• feat(compiler,ts): Phase B — capture lambda body as IR expression tree for IQueryable (#31) #196 — Phase B walker.
• src/Metano.Compiler/Extraction/IrExpressionExtractor.cs (TryCaptureQueryableMeta)
• src/Metano.Compiler/Extraction/IrLinqMapping.cs (TryResolve, IsLinqContainer)