Claude Code 抓蟲閉環：重現 → 定位根因 → 最小修復 → 回歸測試驗證

=== PHASE 1: REPRODUCE === Wrote failing test `cart total with 0-qty item`. Ran `npm test`: ✕ cart total ignores 0-qty lines — expected 100, got NaN Reproduced reliably. === PHASE 2: ISOLATE === Root cause: `lineTotal()` divides by qty to derive unit price, so qty=0 yields NaN which poisons the sum. Symptom is 'total shows NaN'; root cause is the unguarded division at cart.ts:42. (Candidates considered: rounding bug, currency parse — ruled out by the repro.) === PHASE 3: FIX === Guard qty=0 lines before the division at cart.ts:42 — narrowest change that removes the NaN source. === PHASE 4: VERIFY === Repro test now passes. Full suite `npm test`: 214 passing, 0 failing. Summary — Root cause: division by qty=0 at cart.ts:42. Fix: skip 0-qty lines. Regression test: 'cart total with 0-qty item' (committed). Noted but NOT touched: cart.ts also lacks negative-qty handling — separate issue.