feat(v0.17.0/A5): trigram-aware build_match_string + SearchResponse.hint

PR-A 본체. plan Task A4 Step 1c + A5.

- lexical.rs::build_match_string 재설계: whole-phrase + token-AND
  OR-combined, 3자 미만 토큰 drop, 후보 없음 시 None (빈 MATCH
  회피). raw single-quote mode 유지.
- SearchResponse.hint additive — empty result + trimmed < 3 chars
  + non-raw 케이스에 short_query_hint helper 가 set.
- CLI 'kebab search' 가 [hint] stderr 한 줄 (text mode).
- TUI SearchState.short_query_hint + poll_worker stale-aware set
  + fire_search/mark_input_changed reset + dynamic_status 표시.
- docs/wire-schema/v1/search_response.schema.json hint additive.
- 신규 unit tests (lexical 9 PASS, 기존 2 expectation 갱신) +
  통합 회귀 (search_korean: multi_token + mixed, 3 PASS) +
  BM25 snapshot regen (trigram token stream).

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

crates/kebab-search/tests/fixtures/search/lexical/run-1.json

@@ -19,9 +19,9 @@
     "indexed_at": "2024-01-01T00:00:00Z",
     "rank": 1,
     "retrieval": {
-      "fusion_score": 1.4490997273242101e-6,
+      "fusion_score": 1.4615362715630908e-6,
       "lexical_rank": 1,
-      "lexical_score": 1.4490997273242101e-6,
+      "lexical_score": 1.4615362715630908e-6,
       "method": "lexical",
       "vector_rank": null,
       "vector_score": null
@@ -51,9 +51,9 @@
     "indexed_at": "2024-01-01T00:00:00Z",
     "rank": 2,
     "retrieval": {
-      "fusion_score": 9.641424867368187e-7,
+      "fusion_score": 9.207039965986041e-7,
       "lexical_rank": 2,
-      "lexical_score": 9.641424867368187e-7,
+      "lexical_score": 9.207039965986041e-7,
       "method": "lexical",
       "vector_rank": null,
       "vector_score": null