Security: Mitigate lancedb advisory chain (paste)

[saagar210]

Tracking unmaintained advisory for paste proc-macro in LanceDB/DataFusion dependency graph.

Advisories in this track:

• RUSTSEC-2024-0436 (paste)

Current dependency chain evidence:

• lancedb -> lance* / datafusion* -> paste

Planned mitigation path:

1. Monitor lancedb/lance/datafusion upstream for maintained replacement.
2. Evaluate whether vector backend feature scoping can reduce exposure without product regression.
3. Remove waiver once dependency graph no longer includes paste.

Parent issue: #11