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/schema.py

@@ -0,0 +1,50 @@
+"""IMP-33 u2 — AI fallback proposal schema.
+
+Whitelisted proposal kinds (Stage 2 plan):
+  - builder_options_patch     : zone/frame builder option overrides
+  - partial_overrides         : Internal Region / Frame Slot content overrides
+  - slot_mapping_proposal     : restructuring proposal (content unit mapping)
+
+Forbidden output forms (rejected by validator):
+  - mdx_text          (MDX read-only — `feedback_ai_isolation_contract`)
+  - frame_id_change   (V4 rank-1 protected — `feedback_phase_z_spacing_direction`)
+  - raw_html          (HTML structure is code-decided, not AI-generated)
+  - raw_css           (same)
+"""
+from __future__ import annotations
+
+from enum import Enum
+from typing import Any
+
+from pydantic import BaseModel, ConfigDict, Field, field_validator
+
+
+class ProposalKind(str, Enum):
+    BUILDER_OPTIONS_PATCH = "builder_options_patch"
+    PARTIAL_OVERRIDES = "partial_overrides"
+    SLOT_MAPPING_PROPOSAL = "slot_mapping_proposal"
+
+
+FORBIDDEN_KINDS: frozenset[str] = frozenset(
+    {"mdx_text", "frame_id_change", "raw_html", "raw_css"}
+)
+
+
+class AiFallbackProposal(BaseModel):
+    """Single AI fallback proposal (output contract for u4 client)."""
+
+    model_config = ConfigDict(extra="forbid")
+
+    proposal_kind: ProposalKind
+    payload: dict[str, Any] = Field(default_factory=dict)
+    rationale: str = ""
+
+    @field_validator("proposal_kind", mode="before")
+    @classmethod
+    def _reject_forbidden_kind(cls, value: Any) -> Any:
+        if isinstance(value, str) and value in FORBIDDEN_KINDS:
+            raise ValueError(
+                f"proposal_kind={value!r} is forbidden (MDX/frame/raw HTML/CSS "
+                "mutations are not permitted under IMP-33)."
+            )
+        return value