systematic-debugging

# Execute → Bug? Test failure? Unexpected behavior? → **Find root cause first. No fixes without evidence.** 1. Isolate: read error → reproduce → check git diff → drill upward through diagnostic layers: L1 symptom → L2 logic → L3 system → L4 architecture → L5 cross-system contract → L6 platform constraint → L7 spec gap. Stop when no deeper "why" remains OR terminal unactionable (T1-T4). 2. Identify owner: compare with working code → locate canonical owner → flag duplicate owners as a finding 3. Before fixing, run Patch-Shape Triage and Ripple Signal Triage if the candidate fix touches shared/core/cross-module behavior, contract, source-of-truth, fallback, adapter, duplicate owner, producer+consumer, or consumer-side patching. Surface Change Necessity for any new source-code path and non-trivial fixes. Run Minimality Check when the candidate fix adds a new branch, fallback, owner, adapter, or compatibility path. Also run Pre-Edit Complexity Check when the candidate fix touches an overloaded owner or may worsen source complexity. 4. Prove: one hypothesis → smallest reproduction or verification → iterate. A failing test first is required only under explicit `TDD Route: strict`. 3+ failed fixes = question architecture, do not attempt another code fix. After fix, if any symptom persists → differential diagnosis (Phase 4 Step 4bis). 5. Fix: minimal code at canonical owner → targeted verification → Reflection + architecture review → repair + retirement...