feat(p10-r2): V007 trigram migration + design §5.5 + fts diff-check

Task A2 + A3 한 묶음.

migrations/V007__fts_trigram.sql 신규:
- chunks_fts shadow 를 DROP + 재생성 (tokenize = trigram).
- chunks_ai/ad/au trigger 재생성 (V002 와 동일).
- chunks 에서 backfill INSERT — 사용자 re-ingest 불필요, V007 자동.
- V002 는 historical cold-upgrade replay 위해 그대로 유지.

design §5.5 갱신:
- verbatim block 의 tokenize 만 trigram 으로 교체.
- §5.5 본문 상단에 한국어 채택 사유 + trade-off (영어 lexical 변경,
  BM25 분포, 디스크 ~2-10x, contentless 아님) prose 한 단락 추가.

crates/kebab-store-sqlite/tests/fts.rs:
- fts_v002_matches_design_section_5_5_verbatim →
  fts_v007_matches_design_section_5_5_verbatim 으로 rename.
- extract_migration_5_5_verbatim_block() 의 include_str! path 를
  V007__fts_trigram.sql 로 변경. 주석/assertion msg V007 로.
- V002 cold-upgrade test 들 (fts_v002_backfill_*) 은 그대로 유지.

검증: cargo test -p kebab-store-sqlite --test fts → 10/10 PASS
(`fts_v007_matches_design_section_5_5_verbatim` 포함).

Codex round 1/2 의 design §5.5 contentless 정정·trigram tokenizer
채택 사유 명시 발견 반영.

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

migrations/V007__fts_trigram.sql

@@ -0,0 +1,60 @@
+-- V007__fts_trigram.sql — Replace chunks_fts tokenizer: unicode61 → trigram.
+--
+-- Per design §5.5 (chunks_fts virtual table + chunks_ai/ad/au triggers).
+-- The CREATE VIRTUAL TABLE / CREATE TRIGGER block below is reproduced
+-- VERBATIM from `docs/superpowers/specs/2026-04-27-kebab-final-form-design.md`
+-- §5.5; CI diff-checks this against the design doc (test
+-- `fts_v007_matches_design_section_5_5_verbatim` in
+-- `crates/kebab-store-sqlite/tests/fts.rs`).
+--
+-- Tokenizer choice: trigram. Korean is agglutinative — unicode61 tokenizes
+-- whole eojeol (조사·어미 attached) so substring matching fails. trigram
+-- indexes 3-character grams, enabling Korean partial matches. Trade-offs:
+-- DB size grows (~2-10×), English lexical also moves to substring match
+-- (recall↑, precision↓), BM25 score distribution shifts. See
+-- `tasks/HOTFIXES.md` (2026-05-22) and the v0.17.0 design doc.
+--
+-- chunks_fts is a shadow of chunks (NOT contentless — V002 DDL has no
+-- `content=''`); this migration drops the old shadow, recreates it with
+-- the new tokenizer, recreates the sync triggers (verbatim from V002),
+-- and backfills from `chunks`. The `chunks` table and embeddings are
+-- untouched, so users do NOT need to re-ingest after upgrading to
+-- v0.17.0 — the migration is fully automatic.
+
+DROP TRIGGER IF EXISTS chunks_au;
+DROP TRIGGER IF EXISTS chunks_ad;
+DROP TRIGGER IF EXISTS chunks_ai;
+DROP TABLE IF EXISTS chunks_fts;
+
+-- ── §5.5 verbatim block ────────────────────────────────────────────────
+
+CREATE VIRTUAL TABLE chunks_fts USING fts5(
+  chunk_id     UNINDEXED,
+  doc_id       UNINDEXED,
+  heading_path,
+  text,
+  tokenize = 'trigram'
+);
+
+CREATE TRIGGER chunks_ai AFTER INSERT ON chunks BEGIN
+  INSERT INTO chunks_fts(chunk_id, doc_id, heading_path, text)
+  VALUES (new.chunk_id, new.doc_id, new.heading_path_json, new.text);
+END;
+CREATE TRIGGER chunks_ad AFTER DELETE ON chunks BEGIN
+  DELETE FROM chunks_fts WHERE chunk_id = old.chunk_id;
+END;
+CREATE TRIGGER chunks_au AFTER UPDATE ON chunks BEGIN
+  DELETE FROM chunks_fts WHERE chunk_id = old.chunk_id;
+  INSERT INTO chunks_fts(chunk_id, doc_id, heading_path, text)
+  VALUES (new.chunk_id, new.doc_id, new.heading_path_json, new.text);
+END;
+
+-- ── End §5.5 verbatim block ───────────────────────────────────────────
+
+-- One-shot backfill from existing chunks. Mirrors the V002 backfill
+-- pattern — direct INSERT into chunks_fts bypasses chunks_ai trigger
+-- (trigger fires on chunks INSERT, not chunks_fts INSERT), so no
+-- double-insert. Refinery runs V007 exactly once via its bookkeeping
+-- table, so this 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;