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/phase_z2_ai_fallback/router.py

@@ -0,0 +1,89 @@
+"""IMP-33 u7 — AI fallback router (fallback path only).
+
+Composes the IMP-33 fallback flow:
+
+  1. flag gate         (``settings.ai_fallback_enabled`` default OFF)
+  2. V4 route gate     (route must equal ``ai_adaptation_required``)
+  3. cache read        (u6 stub returns ``None`` until IMP-46 lands)
+  4. build prompt      (u3)
+  5. call client       (u4 ``request_proposal``)
+  6. validate          (u5 ``validate_proposal``)
+
+Returns the validated ``AiFallbackProposal``. Save to cache is NOT
+performed here — it is caller-driven AFTER ``visual_check_passed=True``
+AND ``user_approved=True``, per the u6 IMP-46 gate. The router does not
+import ``save_proposal``; this is the structural guarantee that the
+router cannot persist a proposal before the caller's visual + user
+checks (`feedback_artifact_status_naming`).
+
+Guardrails:
+
+* PZ-1 — normal-path AI call count stays 0: flag-off OR route-mismatch
+  short-circuits BEFORE the prompt builder or client are touched.
+* ``feedback_ai_isolation_contract`` — MDX READ-ONLY (u3 enforces in
+  prompt; this module never reads or writes MDX).
+* ``feedback_phase_z_spacing_direction`` — V4 rank-1 protected (u5
+  enforces; router only forwards the contract).
+"""
+from __future__ import annotations
+
+from typing import Any
+
+from src.config import settings
+from src.phase_z2_ai_fallback.cache import read_proposal
+from src.phase_z2_ai_fallback.client import AiFallbackClient
+from src.phase_z2_ai_fallback.prompts import (
+    V4_ROUTE_AI_ADAPTATION,
+    build_ai_fallback_prompt,
+)
+from src.phase_z2_ai_fallback.schema import AiFallbackProposal
+from src.phase_z2_ai_fallback.validate import validate_proposal
+
+
+def route_ai_fallback(
+    *,
+    cache_key: str,
+    v4_result: dict[str, Any],
+    frame_contract: dict[str, Any],
+    frame_visual_html: str,
+    figma_partial_json: dict[str, Any],
+    internal_region: dict[str, Any],
+    mdx_text: str,
+    client: AiFallbackClient | None = None,
+) -> AiFallbackProposal | None:
+    """Route a fallback request through cache → prompt → client → validate.
+
+    Returns ``None`` when the master flag is OFF or when the V4 route is
+    not ``ai_adaptation_required`` — both gates short-circuit BEFORE any
+    prompt/client work, so the normal-path AI call count stays at 0
+    (PZ-1).
+    """
+    if not settings.ai_fallback_enabled:
+        return None
+    route = v4_result.get("route") or v4_result.get("imp05_route_hint")
+    if route != V4_ROUTE_AI_ADAPTATION:
+        return None
+    cached = read_proposal(cache_key)
+    if cached is not None:
+        validate_proposal(
+            cached,
+            frame_contract=frame_contract,
+            internal_region=internal_region,
+        )
+        return cached
+    prompt = build_ai_fallback_prompt(
+        v4_result=v4_result,
+        frame_contract=frame_contract,
+        frame_visual_html=frame_visual_html,
+        figma_partial_json=figma_partial_json,
+        internal_region=internal_region,
+        mdx_text=mdx_text,
+    )
+    active_client = client if client is not None else AiFallbackClient()
+    proposal = active_client.request_proposal(prompt)
+    validate_proposal(
+        proposal,
+        frame_contract=frame_contract,
+        internal_region=internal_region,
+    )
+    return proposal