Boundary: Canonical, reusable 4-phase RCA engine. Invoked by investigate-bug (as step 2 of the end-to-end flow) and by fix-bug (when no bug file exists). Does not write the bug file — that is investigate-bug’s responsibility.

Four phases — do not skip. Update the active specs/bugs/BUG-*.md file at each phase.

1. Reproduce — minimal steps; record environment; capture logs.
2. Isolate — narrow to module/function; binary-search commits or config.
3. Hypothesize — list ranked hypotheses with falsification test each.
4. Verify — run falsification; confirm single root cause; link to fix plan.

HARD GATE — Do not propose a fix until phase 4 confirms one root cause with evidence.

→ verify: BUG_FILE=$(ls -t specs/bugs/BUG-*.md 2>/dev/null | head -1); test -n "$BUG_FILE" && grep -cE "Reproduce|Isolate|Hypothesize|Verify" "$BUG_FILE" | awk '{if($1>=4) print "OK"; else print "INCOMPLETE"}' || echo "MISSING"