feat(#61): IMP-33 AI fallback scaffolding (u1~u11, flag default OFF)

Frame-aware AI fallback module scaffolded under src/phase_z2_ai_fallback/
with master flag ai_fallback_enabled=False; normal-path AI call count
remains 0. AI output constrained to builder_options_patch /
partial_overrides / slot_mapping_proposal; MDX / frame_id / raw HTML /
raw CSS mutations rejected at schema layer. IMP-46 cache gate (cache.py)
raises AiFallbackCacheGateError unless visual_check_passed AND
user_approved. Step 12 wires AI repair after IMP-30 provisional payload
only; Step 17 stays blocked behind IMP-34 / IMP-35 prerequisites.
AST isolation guard forbids fallback package from importing Phase Q /
Kei / pipeline runtime symbols. Docs IMP-17 / IMP-31 bound to runtime
module surface via 11-row structural test pin (test_docs_sync.py) so
drift fails CI.

Tests: 116 fallback / 161 phase_z2 regression / 526 scoped full sweep
all passing. Existing pre-IMP-33 fixture issue in scripts/test_phase_t_*
remains untouched (out of scope).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

docs/architecture/IMP-31-GATE-AUDIT.md

@@ -28,7 +28,7 @@ Anchor pin: `tests/orchestrator_unit/test_imp17_comment_anchor.py`. Synced in [`
 | 2 | B4 frame_selection evidence integration complete | **NOT CLEAR** (⚠ partial) | [`PHASE-Z-PIPELINE-STATUS-BOARD.md`](PHASE-Z-PIPELINE-STATUS-BOARD.md):48 Step 9 ⚠ partial; :82 "B4 frame_selection 의 V4 evidence 미통합"; :126 (j) ❌ pending. |
 | 3 | IMP-04 catalog expansion + IMP-05 V4 fallback live | **AMBIGUOUS** | `templates/phase_z2/catalog/frame_contracts.yaml` = 11 `template_id:` entries vs 32 target. IMP-05 V4 rank-2/3 fallback selector logic live, but catalog coverage gates real semantics. |
 
-**Verdict**: gate **NOT CLEAR**. Runtime AI adaptation remains design-only. `src/phase_z2_ai_fallback/` = declaration-only path (not created this cycle).
+**Verdict**: gate **NOT CLEAR**. Runtime AI adaptation remains gated. `src/phase_z2_ai_fallback/` = **scaffolded under IMP-33** (#61, Stage 3 u1~u11); module created, but `settings.ai_fallback_enabled` defaults to `False` (u1) so normal-path AI call count remains 0 (PZ-1). Runtime engagement still requires the 3-condition AND gate above.
 
 ## Issue-body axis verdict
 
@@ -47,13 +47,13 @@ Anchor pin: `tests/orchestrator_unit/test_imp17_comment_anchor.py`. Synced in [`
 
 ## Out of scope (this cycle)
 
-Runtime AI module, `src/phase_z2_ai_fallback/` directory creation, prompt implementation, `candidate_evidence` schema change, Phase Q file mutation, Kei API reuse, frontend zone override (IMP-29 scope), IMP-30 invariant change, `calculate_fit` migration.
+Runtime AI consumer enablement (flag default OFF), `candidate_evidence` schema change, Phase Q file mutation, Kei API reuse, frontend zone override (IMP-29 scope), IMP-30 invariant change, `calculate_fit` migration. Note: `src/phase_z2_ai_fallback/` directory scaffold itself was created under IMP-33 (#61, Stage 3 u1~u11) — see [`IMP-17-CARVE-OUT.md`](IMP-17-CARVE-OUT.md) §"Runtime module surface".
 
-## Future activation path (declaration only)
+## Future activation path
 
 When the 3-condition AND gate clears (User GO ∧ B4 V4 evidence integrated ∧ catalog 32/32 + IMP-05 V4 fallback live):
 
-- Runtime AI module path = `src/phase_z2_ai_fallback/` (not created this cycle).
+- Runtime AI module path = `src/phase_z2_ai_fallback/` (scaffolded under IMP-33; flag default OFF until gate clears).
 - Provider = Anthropic API only. Prompt design starts fresh (no Phase Q `EDITOR_PROMPT` import).
 - Output granularity = content_object → Internal Region / Frame Slot placement proposal. Frame / layout / zone topology selection remains deterministic.
 - Activation tracker = this issue (#40, IMP-31). No new IMP ID issued.