feat(internal): add index creation retry logic with exponential backoff (BL-048)

[tryweb]

Summary

• BL-048 implementation complete: LanceDB index creation now has retry logic with exponential backoff
• Handles transient LanceDB index creation conflicts gracefully
• Adds idempotency check using table.listIndices() before attempting index creation
• Adds structured logging and retry count tracking for observability

Verification

• openspec validate: passed
• Unit tests: 32/32 passed
• Documentation: CHANGELOG.md updated

Changes

• src/store.ts: Added createVectorIndexWithRetry() and createFtsIndexWithRetry() with exponential backoff (500ms → 1s → 2s)
• src/store.ts: Extended LanceTable type to include listIndices()
• src/store.ts: Extended getIndexHealth() to return vectorRetries and ftsRetries
• docs/backlog.md: Updated BL-048 status to done
• docs/roadmap.md: Updated BL-048 to DONE v0.6.1

Notes

• This is an internal-only change - no new user-facing capabilities
• Vector index may fail in Docker test environment (empty table), but fallback to in-memory search works correctly
• Retry logic verified via test output showing: Vector index creation failed (attempt 1/3)... Retrying in 500ms...

Test Output Sample

[store] Vector index creation failed (attempt 1/3): lance error: ... Retrying in 500ms...
[store] Vector index creation failed (attempt 2/3): ... Retrying in 1000ms...
[store] Vector index creation failed after 3 attempts: ... Falling back to in-memory search.
[store] FTS index created successfully on attempt 1