refactor(core): split search cache modules

[xingkaixin]

Summary

• move structured query parsing and FTS token escaping into search-query-parser.ts
• move search index state, SQLite writes, and FTS rebuild orchestration into search-index-writer.ts
• retain search.ts as the query reader and stable compatibility export surface
• move parser characterization coverage next to its implementation

Impact

This is an internal locality refactor. Search exports, SQL parameter order, FTS behavior, result ranking, and index synchronization behavior remain unchanged. No adapter or speculative abstraction was introduced around SQLite.

Validation

• pnpm format
• pnpm lint
• pnpm format:check
• pnpm build
• pnpm test (662 tests)
• pnpm test:e2e (6 tests)
• pnpm bench:perf --target auto --iterations 1

Closes CS-46