feat: incremental FTS5 for SQLite — eliminate full index rebuilds
Replace drop-and-rebuild FTS strategy with incremental external content
FTS5 tables for SQLite. Records are indexed/deindexed inline during
insertRecord/deleteRecord. Periodic and post-backfill rebuilds are
skipped for SQLite. DuckDB retains its existing behavior.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>