Problem

FTS5 unicode61 (porter) tokenizer cannot segment CJK ideographs. When a query contains Chinese/Japanese/Korean characters, searchSummaries() falls back to a LIKE path with AND logic via buildLikeSearchPlan().

The AND logic fails when the user's phrasing doesn't exactly match the summary text:

This affects all CJK users — Chinese, Japanese, and Korean — and is the primary reason lcm_expand_query returns zero candidates for CJK queries (even when matching summaries exist).

Western/Latin users are less affected because space-separated words align naturally with the \S+ regex tokenizer.

Solution

1. Trigram FTS table (summaries_fts_cjk)

A new FTS5 virtual table with tokenize='trigram' that indexes every 3-character substring. This enables native CJK substring matching via FTS5 MATCH.

2. searchCjkTrigram() — primary CJK search path

• Splits CJK segments into overlapping 4-char chunks
• Combines with OR semantics via FTS5 MATCH
• Non-CJK tokens (English words, version numbers) searched in existing porter FTS table
• Results unioned and sorted by recency

3. searchLikeCjk() — fallback when trigram table unavailable

• Splits CJK text into bigrams (2-char sliding window)
• Uses LIKE with OR instead of AND
• Ensures partial matches return results even without the trigram table

4. Auto-migration

• Creates summaries_fts_cjk and backfills from existing summaries on first run
• New summaries indexed on saveSummary()
• Graceful degradation: if table doesn't exist, falls through to LIKE OR

Testing

Tested on 4 machines with Chinese query workloads:

Before: "端到端测试结果" → 0 candidates (AND logic, exact match required)
After:  "端到端测试结果" → correct matches (trigram OR, substring matching)

Also verified:

• Exact CJK matches still work
• Mixed CJK + Latin queries (e.g. "lossless-claw v0.5.2 端到端测试") return correct results
• Non-CJK queries unaffected (existing porter FTS path unchanged)
• No regressions on lcm_grep or lcm_describe

Files changed

• src/db/migration.ts — add summaries_fts_cjk virtual table creation
• src/store/summary-store.ts — add searchCjkTrigram(), searchLikeCjk(), update CJK routing, index new summaries into trigram table

Related: #208 (search path for lcm_expand_query candidate resolution)