D3 — SQLite: modernc.org/sqlite planned, not yet introduced

For the FTS5 trigram index, modernc.org/sqlite is preferred (pure Go, no cgo). To be introduced when the in-memory index is no longer enough (large KBs, semantic search in SQLite). (Superseded by D32/D43: introduced as internal/sqlindex — FTS5 trigram + embedding cache, best-effort with in-memory fallback.)