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/validate.py

@@ -0,0 +1,83 @@
+"""IMP-33 u5 — AI fallback proposal validator (fallback path only).
+
+Defence-in-depth layer between the u4 client output (already u2-schema-valid)
+and the caller. Adds the four Stage 2 guards that u2 cannot express purely at
+the schema level:
+
+  1. builder-options whitelist (BUILDER_OPTIONS_PATCH may only touch keys
+     already declared in ``frame_contract.payload.builder_options``).
+  2. dropped-slot guard (PARTIAL_OVERRIDES / SLOT_MAPPING_PROPOSAL must keep
+     every declared ``sub_zones[*].id`` populated — text/table/image/details
+     slots cannot disappear; `feedback_ai_isolation_contract`).
+  3. frame-swap guard (no ``frame_id`` mutation inside payload — V4 rank-1
+     protected; `feedback_phase_z_spacing_direction`).
+  4. Internal Region containment (``payload.region_id`` must match the
+     declared Internal Region id when present).
+"""
+from __future__ import annotations
+
+from typing import Any
+
+from src.phase_z2_ai_fallback.schema import AiFallbackProposal, ProposalKind
+
+
+class AiFallbackValidationError(ValueError):
+    """Raised when a proposal violates an IMP-33 u5 guard."""
+
+
+_SLOT_KINDS = (ProposalKind.PARTIAL_OVERRIDES, ProposalKind.SLOT_MAPPING_PROPOSAL)
+
+
+def validate_proposal(
+    proposal: AiFallbackProposal,
+    *,
+    frame_contract: dict[str, Any],
+    internal_region: dict[str, Any] | None = None,
+) -> None:
+    """Validate an AI fallback proposal against the active frame contract.
+
+    Raises ``AiFallbackValidationError`` on any guard violation. Returns
+    ``None`` on success — caller is responsible for downstream application.
+    """
+    AiFallbackProposal.model_validate(proposal.model_dump())
+
+    payload = proposal.payload
+    frame_id = frame_contract.get("frame_id")
+    if "frame_id" in payload and payload["frame_id"] != frame_id:
+        raise AiFallbackValidationError(
+            f"frame-swap guard: payload.frame_id={payload['frame_id']!r} "
+            f"differs from contract frame_id={frame_id!r}; V4 rank-1 is locked."
+        )
+
+    if proposal.proposal_kind is ProposalKind.BUILDER_OPTIONS_PATCH:
+        declared = (frame_contract.get("payload") or {}).get("builder_options") or {}
+        unknown = set(payload.keys()) - set(declared.keys())
+        if unknown:
+            raise AiFallbackValidationError(
+                f"builder whitelist: keys {sorted(unknown)} not in "
+                f"frame_contract.payload.builder_options {sorted(declared)}."
+            )
+
+    if proposal.proposal_kind in _SLOT_KINDS:
+        declared_slot_ids = [z.get("id") for z in (frame_contract.get("sub_zones") or [])]
+        slots = payload.get("slots")
+        if not isinstance(slots, dict):
+            raise AiFallbackValidationError(
+                "dropped-slot guard: PARTIAL_OVERRIDES / SLOT_MAPPING_PROPOSAL "
+                "payload MUST include a 'slots' mapping."
+            )
+        missing = [sid for sid in declared_slot_ids if sid not in slots]
+        if missing:
+            raise AiFallbackValidationError(
+                f"dropped-slot guard: declared slots {missing} are absent "
+                "from payload.slots (text/table/image/details must remain populated)."
+            )
+
+    region_id = payload.get("region_id")
+    if region_id is not None and internal_region is not None:
+        declared_region_id = internal_region.get("id")
+        if region_id != declared_region_id:
+            raise AiFallbackValidationError(
+                f"Internal Region containment: payload.region_id={region_id!r} "
+                f"differs from internal_region.id={declared_region_id!r}."
+            )