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>

docs/architecture/IMP-17-CARVE-OUT.md

@@ -51,5 +51,5 @@ Phase Q `content_editor.py` 는 **Archive Candidate** ([`PHASE-Q-AUDIT.md`](PHAS
 | Step 12 entry | `src.phase_z2_ai_fallback.step12.gather_step12_ai_repair_proposals` — IMP-30 provisional gate (`not_provisional` skip) AND reject gate (`design_reference_only_no_ai` skip) AND non-AI route catch-all run BEFORE `route_ai_fallback`. |
 | Step 17 entry | `src.phase_z2_ai_fallback.step17.gather_step17_ai_repair_proposals` — STRUCTURALLY BLOCKED. Every unit returns `skip_reason="step17_ai_blocked_imp_34_35_prerequisites_missing"`. Module does NOT import `route_ai_fallback` / `AiFallbackClient` / `anthropic`. |
 | Cascade order | `src.phase_z2_ai_fallback.step17.OVERFLOW_CASCADE_ORDER = (DETERMINISTIC, POPUP, AI_REPAIR, USER_OVERRIDE)` — single source of truth for Step 17 consumers. Aligns with line 16 of this doc. |
-| IMP-46 cache gate | `src.phase_z2_ai_fallback.cache.save_proposal(..., visual_check_passed, user_approved)` raises `AiFallbackCacheGateError` unless BOTH gates are True; storage backend then raises `NotImplementedError` (IMP-46 marker). `read_proposal` returns `None` until IMP-46 lands a backend. |
+| IMP-46 cache gate | `src.phase_z2_ai_fallback.cache.save_proposal(..., visual_check_passed, user_approved, auto_cache=False)` raises `AiFallbackCacheGateError` unless `visual_check_passed=True` AND (`user_approved=True` OR `auto_cache=True`). Persistent JSON backend at `data/frame_cache/{frame_id}/{signature_hash}.json` (u2); cache key = structural signature over 8 axes (u1+u4); read-side fingerprint invalidation via `read_proposal(..., fingerprints=...)` strict equality (u3); `--auto-cache` CLI flag + `settings.ai_fallback_auto_cache` (default `False`) bypasses ONLY the `user_approved` gate (u5); repo root tracked via `data/frame_cache/.gitkeep` with cached payloads git-ignored (u6). `read_proposal` returns `None` on missing / corrupt / fingerprint-mismatched entries — cache is a hint, never a hard dependency. |
 | AST isolation | `tests/phase_z2_ai_fallback/test_ast_isolation.py` parses every `*.py` under `src/phase_z2_ai_fallback/` and forbids Phase Q runtime / Kei client / `src.phase_z2_*` (non-fallback) imports. Whitelist = `src.config` + intra-package + stdlib + `anthropic` + `pydantic`. |