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/step12.py

@@ -0,0 +1,141 @@
+"""IMP-33 u8 — Step 12 AI repair wiring (IMP-30 provisional units only).
+
+Phase Z Step 12 = slot_payload (the runtime "light_edit / restructure" surface
+where AI-assisted frame-aware adaptation is allowed per IMP-17 carve-out).
+This module is the only call site that pipes Phase Z composition units into
+``src.phase_z2_ai_fallback.router.route_ai_fallback``. Two structural gates
+preserve the AI isolation contract:
+
+* IMP-30 provisional gate — units with ``provisional=False`` are skipped
+  before any route classification. AI repair is reserved for first-render
+  invariant survivors (no rank-1 V4 evidence, recovered as provisional).
+* Reject gate — units whose V4 label maps to ``design_reference_only``
+  (``reject``) are skipped with ``skip_reason="design_reference_only_no_ai"``.
+  Reject path is design reference only — never an AI call.
+
+Combined with the u7 router's flag-off + route-gate short-circuits, the
+default Phase Z run path performs zero AI calls (PZ-1). Save to cache is
+NOT performed here — that is the caller's responsibility AFTER
+``visual_check_passed=True`` AND ``user_approved=True`` (u6 IMP-46 gate).
+"""
+from __future__ import annotations
+
+from typing import Any, Callable, Iterable
+
+from src.phase_z2_ai_fallback.router import route_ai_fallback
+
+
+_AI_ADAPTATION_ROUTE = "ai_adaptation_required"
+_DESIGN_REFERENCE_ROUTE = "design_reference_only"
+
+
+def gather_step12_ai_repair_proposals(
+    units: Iterable[Any],
+    *,
+    route_for_label: Callable[[str | None], str | None],
+    get_contract_fn: Callable[[str], dict | None],
+    frame_visual_loader: Callable[[str], str],
+    figma_partial_loader: Callable[[str], dict] | None = None,
+    internal_region_lookup: Callable[[Any], dict] | None = None,
+    mdx_text_loader: Callable[[Any], str] | None = None,
+) -> list[dict]:
+    """Return one record per unit describing the Step 12 AI repair decision.
+
+    The record schema is stable across all gate decisions so the Step 12
+    artifact consumer can rely on a single shape:
+
+        {
+          "unit_index": int,
+          "source_section_ids": list[str],
+          "frame_template_id": str,
+          "label": str | None,
+          "route_hint": str | None,
+          "provisional": bool,
+          "ai_called": bool,
+          "skip_reason": str | None,
+          "proposal": dict | None,
+          "error": str | None,
+        }
+
+    ``ai_called`` is True only when ``route_ai_fallback`` was invoked AND
+    returned a proposal OR raised. Flag-off / route-mismatch returns
+    ``None`` from the router and is surfaced as ``ai_called=False`` with
+    ``skip_reason="router_short_circuit"`` so the caller can distinguish
+    "router decided not to run" from "router ran and returned a proposal".
+    """
+    records: list[dict] = []
+    for index, unit in enumerate(units):
+        label = getattr(unit, "label", None)
+        route_hint = route_for_label(label)
+        record: dict = {
+            "unit_index": index,
+            "source_section_ids": list(getattr(unit, "source_section_ids", []) or []),
+            "frame_template_id": getattr(unit, "frame_template_id", None),
+            "label": label,
+            "route_hint": route_hint,
+            "provisional": bool(getattr(unit, "provisional", False)),
+            "ai_called": False,
+            "skip_reason": None,
+            "proposal": None,
+            "error": None,
+        }
+        if not record["provisional"]:
+            record["skip_reason"] = "not_provisional"
+            records.append(record)
+            continue
+        if route_hint == _DESIGN_REFERENCE_ROUTE:
+            record["skip_reason"] = "design_reference_only_no_ai"
+            records.append(record)
+            continue
+        if route_hint != _AI_ADAPTATION_ROUTE:
+            record["skip_reason"] = f"route_not_ai_adaptation:{route_hint}"
+            records.append(record)
+            continue
+
+        template_id = record["frame_template_id"] or ""
+        frame_contract = get_contract_fn(template_id) or {}
+        frame_visual_html = frame_visual_loader(template_id)
+        figma_partial_json = (
+            figma_partial_loader(template_id) if figma_partial_loader is not None else {}
+        )
+        internal_region = (
+            internal_region_lookup(unit) if internal_region_lookup is not None else {}
+        )
+        mdx_text = (
+            mdx_text_loader(unit)
+            if mdx_text_loader is not None
+            else (getattr(unit, "raw_content", "") or "")
+        )
+        cache_key = "::".join(
+            [template_id, ",".join(sorted(record["source_section_ids"]))]
+        )
+        v4_result = {
+            "route": route_hint,
+            "label": label,
+            "frame_id": getattr(unit, "frame_id", None),
+            "rank": getattr(unit, "v4_rank", None),
+            "cardinality": None,
+        }
+        try:
+            proposal = route_ai_fallback(
+                cache_key=cache_key,
+                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,
+            )
+        except Exception as exc:  # noqa: BLE001 — record + continue, no AI re-raise
+            record["ai_called"] = True
+            record["error"] = f"{type(exc).__name__}: {exc}"
+            records.append(record)
+            continue
+        if proposal is None:
+            record["skip_reason"] = "router_short_circuit"
+            records.append(record)
+            continue
+        record["ai_called"] = True
+        record["proposal"] = proposal.model_dump()
+        records.append(record)
+    return records