fix: harden document fallback paths

[lzehrung]

Summary

• harden markdown document-link extraction against malformed and nested link/image syntax without creating false edges
• enforce native:on availability consistently across graph-only, cached, and symbol/detail fallback paths
• add regressions covering malformed markdown, graph-only docs, and required-native behavior

Testing

• npm run test:fast tests/document-links.test.ts tests/fallback-import-extraction.test.ts tests/agent-orient.test.ts tests/native-fallback-contract.test.ts