fix: make exact symbol lookup abstain

[Helweg]

Closes #318

Summary

• Resolve explicit exact symbols from the active catalog rather than semantic ranking.
• Preserve scope filters and return clear not_found / ambiguous structured metadata.
• Expose resolution metadata through MCP for lookup, context, and graph tools.
• Add generated/vendor and stale active-catalog regression coverage.

Validation

• npm run build
• npm run lint
• npm run test:run (102 files, 1,604 tests)
• Built stdio MCP smoke: exact lookup/context misses and graph/path unresolved endpoints return structured not_found evidence without semantic fallback.