D32 — Search index persisted on SQLite (internal/sqlindex)

Decision. Introduced the internal/sqlindex package with the modernc.org/sqlite dependency (pure-Go driver, no cgo — resolves D3). Persists in <root>/.cartographer/index.db (gitignored via .cartographer/): - concepts(id, content_hash, body) + FTS5 virtual table concepts_fts with tokenize='trigram' → substring keyword search (overcomes the "whole words only" limit of the in-memory inverted index, D12). - embeddings(id, content_hash, model, vec BLOB) with the vector serialized as little-endian float64. (This half is superseded by D135: the table is no longer created, read or written.) EmbeddingFresh(id, hash) enables the per-content-hash cache: index_rebuild recomputes the Ollama embedding only for concepts whose hash changed (previously everything was re-embedded at every rebuild/startup).

Wiring: RegisterKBToolsWithSQLIndex (called by main.go when the embedder is active) registers search/index_rebuild variants that use sqlIdx if non-nil. main.go opens the per-KB DB best-effort: if Open fails (or FTS5 is unavailable) it logs to stderr and proceeds with the existing in-memory path. The previous in-memory functions and tools (RegisterKBToolsWithEmbed, toolSearchWithEmbed) remain for backward compatibility and for the sqlIdx == nil case. (D36 update: unified registration in RegisterKBTools(s, k, Deps); FTS5 is now active even without an embedder.)

Rationale. The in-memory index is rebuilt at every startup by walking all concepts and — when the embedder is active — re-calling Ollama for each concept: expensive on large KBs and at every restart. SQLite persistence with a per-content-hash cache makes the embedding cost proportional to the changed concepts, and FTS5 trigram provides real substring search. modernc.org/sqlite is pure-Go (no cgo, no C toolchain) → consistent with binary portability. The best-effort fallback guarantees no environment regresses: without the DB, behavior is identical to before.