Adds FTS5 lexical index for chunks per design §5.5: chunks_fts virtual
table (unicode61 remove_diacritics 2 tokenizer, contentless w/ UNINDEXED
chunk_id+doc_id) plus chunks_ai/chunks_ad/chunks_au triggers that mirror
every chunks mutation into chunks_fts inside the host transaction.

V002 ships the verbatim §5.5 SQL block plus a one-shot backfill INSERT
so existing P1 databases gain searchability without re-ingest. Refinery
bookkeeping makes double-apply naturally idempotent.

Adds rebuild_chunks_fts(&Connection) escape hatch for kb index
--rebuild-fts (CLI wiring deferred to a later task). Uses SAVEPOINT
instead of Transaction so callers can invoke from inside an outer
transaction; WAL serializes writers so no DELETE/INSERT race vs.
concurrent chunks mutators is possible.

Tests (10): V001-only → V002 cold-upgrade backfill (literal path),
chunks_ai/ad/au trigger sync, MATCH-token verification, rebuild
idempotency, drift recovery, double-run no-op, V002 ↔ design §5.5
verbatim diff guard (anchored extraction from both files), WAL/SHM
release on store drop. All 185 workspace tests pass.

Allowed deps respected (kb-core, kb-config, rusqlite, refinery — no
new deps). FTS query implementation deferred to p2-2 (lexical-retriever).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

@@ -0,0 +36,4 @@
+/// correctly. A panic inside the DELETE/INSERT closure leaks the
+/// savepoint name on the connection until the connection is dropped;
+/// that's acceptable because the next caller's `SAVEPOINT
+/// rebuild_chunks_fts` legally shadows the leaked one.

@@ -0,0 +55,4 @@
+        )
+        .context("repopulate chunks_fts from chunks")?;
+        Ok(())
+    })();

@@ -0,0 +62,4 @@
+///
+/// This is the literal cold-upgrade path: V001-shipped database, V002
+/// applied on top, existing chunks become searchable without re-ingest.
+/// The trigger-based mirror (chunks_ai) is covered by the §2 tests.

@@ -0,0 +319,4 @@
+/// blank-line counts while still catching token-level changes.
+fn normalize_ws(s: &str) -> String {
+    s.split_whitespace().collect::<Vec<_>>().join(" ")
+}

@@ -0,0 +368,4 @@
+        .rfind("END;")
+        .expect("§5.5 FTS slice must terminate with `END;`");
+    fts_slice[..last_end + "END;".len()].to_string()
+}

@@ -0,0 +475,4 @@
+    if db_path.exists() {
+        std::fs::remove_file(&db_path)
+            .expect("main DB file should be removable after store drop");
+    }

@@ -0,0 +44,4 @@
+-- runs V002 exactly once via its bookkeeping table, so this INSERT is
+-- naturally idempotent across restarts.
+INSERT INTO chunks_fts(chunk_id, doc_id, heading_path, text)
+  SELECT chunk_id, doc_id, heading_path_json, text FROM chunks;