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>

src/config.py

@@ -14,6 +14,18 @@ class Settings(BaseSettings):
     slide_width: int = 1280
     slide_height: int = 720
 
+    # IMP-33 u1 — AI fallback policy. Fallback-path only; normal path AI=0.
+    # Defaults locked by Stage 2 plan; do NOT inline literals downstream.
+    ai_fallback_enabled: bool = False
+    ai_fallback_model: str = "claude-opus-4-6-20250415"
+    ai_fallback_timeout_s: float = 60.0
+    ai_fallback_max_retries: int = 3
+    ai_fallback_backoff_base_s: float = 1.0
+    ai_fallback_backoff_cap_s: float = 8.0
+    ai_fallback_backoff_jitter: float = 0.3
+    ai_fallback_budget_per_run: int = 10
+    ai_fallback_circuit_breaker_threshold: int = 5
+
     model_config = {"env_file": ".env", "env_file_encoding": "utf-8"}