feat(#76): IMP-47B reject-as-AI-adaptation activation (u1~u13 backend + tests)

- u1~u9: AI fallback infrastructure (router/prompts/schema/validator) + Step 12 hook
- u10: e2e reject chain (writes final.html with AI-repaired slot, full coverage)
- u11: frontend wiring deferred to follow-up commit (split from IMP-41 hunks)
- u12: coverage_invariant guard
- u13: cache save gate (visual_check PASS + user_approved/auto_cache) — Codex #22 verified

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

.gitignore

@@ -8,7 +8,11 @@ dist/
 build/
 .venv/
 node_modules/
-data/
+data/*
+# IMP-46 u6 — track only the frame_cache directory marker; cached payloads stay ignored.
+!data/frame_cache/
+data/frame_cache/*
+!data/frame_cache/.gitkeep
 
 # session workspace (push X — 작업 흐름 trace, 사용자 결정 2026-05-08)
 forex/