Harden MIR validation against nested typed-nil values

[itsfuad]

Summary

• Reject nested typed-nil MIR values with validation errors instead of panicking.
• Validate MIR at direct LLVM entry and prevent backend phase advancement when emission fails.
• Add validator, LLVM, and pipeline regression coverage.
• Update IR/backend architecture docs and remove architecture-audit-masterprompt.md.

Production ownership

• mir.Module.Validate remains canonical owner of backend-independent MIR validation.
• Nested guards reuse typednil.IsNil.
• Pipeline and direct LLVM callers share canonical validator; no new production symbols or helpers.

Design and behavior

• MIR rejected by Module.Validate() produces ErrInvalidEvidence and empty LLVM output.
• Pipeline remains at MIR when emission reports errors or returns empty output.
• Plain-nil return values and omitted payloads for payloadless variants preserve existing semantics.
• Direct-entry revalidation protects callers that bypass pipeline.

Validation

Passed during implementation:

CCACHE_DISABLE=1 go test -count=1 ./internal/ir/mir ./internal/pipeline ./internal/backend/llvm
CCACHE_DISABLE=1 go test -count=1 ./...
CCACHE_DISABLE=1 go test -race -count=1 ./...
go vet ./...

Formatting clean. Current branch diff check passed:

git diff --check origin/main...HEAD

Risks and follow-up

• Pipeline regression exercises MIR-to-backend rejection; earlier post-lowering validation remains intact.
• Revalidation at both entry boundaries intentionally uses same canonical implementation.
• Linked follow-up issues: none.