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>

docs/wire-schema/v1/search_response.schema.json

@@ -29,6 +29,10 @@
           }
         }
       }
+    },
+    "hint": {
+      "type": "string",
+      "description": "v0.17.0 A5 Step 4b: advisory string set when the empty hit list is likely due to a query shorter than the FTS5 trigram tokenizer's 3-char minimum. Field is omitted when no advisory applies. Raw FTS5 mode ('...') opts out. MCP / agent consumers should surface this so users understand the empty result rather than retrying the same short query."
     }
   }
 }