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>

tests/phase_z2_ai_fallback/test_schema.py

@@ -0,0 +1,46 @@
+"""IMP-33 u2 — AiFallbackProposal schema tests.
+
+Scope (Stage 2 plan, u2):
+  - Whitelisted proposal_kind values are accepted.
+  - Forbidden output forms are rejected: mdx_text / frame_id_change / raw_html / raw_css.
+  - extra fields outside the declared schema are rejected (MDX read-only signal).
+"""
+from __future__ import annotations
+
+import pytest
+from pydantic import ValidationError
+
+from src.phase_z2_ai_fallback import AiFallbackProposal, ProposalKind
+
+
+@pytest.mark.parametrize(
+    "kind_value",
+    [
+        "builder_options_patch",
+        "partial_overrides",
+        "slot_mapping_proposal",
+    ],
+)
+def test_whitelisted_proposal_kinds_accepted(kind_value: str) -> None:
+    proposal = AiFallbackProposal(proposal_kind=kind_value)
+    assert proposal.proposal_kind == ProposalKind(kind_value)
+
+
+@pytest.mark.parametrize(
+    "forbidden",
+    ["mdx_text", "frame_id_change", "raw_html", "raw_css"],
+)
+def test_forbidden_proposal_kinds_rejected(forbidden: str) -> None:
+    with pytest.raises(ValidationError):
+        AiFallbackProposal(proposal_kind=forbidden)
+
+
+def test_unknown_proposal_kind_rejected() -> None:
+    with pytest.raises(ValidationError):
+        AiFallbackProposal(proposal_kind="something_else")
+
+
+def test_extra_fields_rejected() -> None:
+    """`extra=forbid` keeps the AI from smuggling raw_html/mdx_text alongside a valid kind."""
+    with pytest.raises(ValidationError):
+        AiFallbackProposal(proposal_kind="partial_overrides", raw_html="<div/>")