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>

crates/kebab-store-sqlite/tests/fts.rs

@@ -370,17 +370,19 @@ fn extract_design_5_5_fts_block() -> String {
     fts_slice[..last_end + "END;".len()].to_string()
 }
 
-/// Extract the §5.5 verbatim block from the V002 migration, between the
-/// `── §5.5 verbatim block ──` anchor markers the file already carries.
+/// Extract the §5.5 verbatim block from the V007 migration (replaced V002
+/// 's unicode61 tokenizer with trigram — V002 stays in place for
+/// historical cold-upgrade replay but V007 is now the source of truth),
+/// between the `── §5.5 verbatim block ──` anchor markers V007 carries.
 fn extract_migration_5_5_verbatim_block() -> String {
-    let migration = include_str!("../../../migrations/V002__fts.sql");
+    let migration = include_str!("../../../migrations/V007__fts_trigram.sql");
     // The opening anchor line ends with `── §5.5 verbatim block ─...`.
     let open_marker = "§5.5 verbatim block";
     let close_marker = "End §5.5 verbatim block";
 
     let open_idx = migration
         .find(open_marker)
-        .expect("V002 must carry the `§5.5 verbatim block` opening anchor");
+        .expect("V007 must carry the `§5.5 verbatim block` opening anchor");
     let after_open_line = open_idx
         + migration[open_idx..]
             .find('\n')
@@ -389,7 +391,7 @@ fn extract_migration_5_5_verbatim_block() -> String {
 
     let close_idx = migration[after_open_line..]
         .find(close_marker)
-        .expect("V002 must carry the `End §5.5 verbatim block` closing anchor")
+        .expect("V007 must carry the `End §5.5 verbatim block` closing anchor")
         + after_open_line;
     // Walk back from the close marker to the start of its comment line.
     let close_line_start = migration[..close_idx]
@@ -400,12 +402,14 @@ fn extract_migration_5_5_verbatim_block() -> String {
     migration[after_open_line..close_line_start].to_string()
 }
 
-/// CI diff guard: the §5.5 block in `migrations/V002__fts.sql` must
-/// match the design doc verbatim (whitespace-normalized). If the
-/// design doc moves the section, renames the heading, or edits the
-/// SQL, this test fails first. Same for migration drift.
+/// CI diff guard: the §5.5 block in `migrations/V007__fts_trigram.sql`
+/// must match the design doc verbatim (whitespace-normalized). V007
+/// replaced V002 's unicode61 tokenizer with trigram (2026-05-23).
+/// V002 stays in place for historical replay of cold-upgrade paths
+/// but is no longer compared against the design doc — V007 is now
+/// the source of truth.
 #[test]
-fn fts_v002_matches_design_section_5_5_verbatim() {
+fn fts_v007_matches_design_section_5_5_verbatim() {
     let design = extract_design_5_5_fts_block();
     let migration_block = extract_migration_5_5_verbatim_block();
 
@@ -428,7 +432,7 @@ fn fts_v002_matches_design_section_5_5_verbatim() {
     let migration_n = normalize_ws(&migration_block);
     assert_eq!(
         design_n, migration_n,
-        "V002__fts.sql §5.5 block must match design doc §5.5 verbatim \
+        "V007__fts_trigram.sql §5.5 block must match design doc §5.5 verbatim \
          (whitespace-normalized). If you intentionally changed one, \
          update the other in the same commit."
     );