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_cache.py

@@ -0,0 +1,90 @@
+"""IMP-33 u6 — AI fallback cache gate tests.
+
+Verifies the IMP-46 gate contract:
+  * ``read_proposal`` is a stub (returns None until IMP-46).
+  * ``save_proposal`` enforces both gates before any write attempt.
+  * Storage itself raises NotImplementedError (IMP-46 marker).
+"""
+from __future__ import annotations
+
+import pytest
+
+from src.phase_z2_ai_fallback.cache import (
+    AiFallbackCacheGateError,
+    read_proposal,
+    save_proposal,
+)
+from src.phase_z2_ai_fallback.schema import AiFallbackProposal, ProposalKind
+
+
+def _proposal() -> AiFallbackProposal:
+    return AiFallbackProposal(
+        proposal_kind=ProposalKind.BUILDER_OPTIONS_PATCH,
+        payload={"item_parser": "bullet_v2"},
+        rationale="u6-test",
+    )
+
+
+def test_read_proposal_returns_none_for_any_key():
+    assert read_proposal("frame=foo|cardinality=3") is None
+
+
+def test_read_proposal_rejects_empty_key():
+    with pytest.raises(ValueError):
+        read_proposal("")
+
+
+def test_save_rejects_when_visual_check_failed():
+    with pytest.raises(AiFallbackCacheGateError) as exc:
+        save_proposal(
+            "k", _proposal(), visual_check_passed=False, user_approved=True
+        )
+    assert "visual_check_passed" in str(exc.value)
+
+
+def test_save_rejects_when_user_not_approved():
+    with pytest.raises(AiFallbackCacheGateError) as exc:
+        save_proposal(
+            "k", _proposal(), visual_check_passed=True, user_approved=False
+        )
+    assert "user_approved" in str(exc.value)
+
+
+def test_save_rejects_when_both_gates_false():
+    with pytest.raises(AiFallbackCacheGateError):
+        save_proposal(
+            "k", _proposal(), visual_check_passed=False, user_approved=False
+        )
+
+
+def test_save_raises_not_implemented_when_both_gates_pass():
+    with pytest.raises(NotImplementedError) as exc:
+        save_proposal(
+            "k", _proposal(), visual_check_passed=True, user_approved=True
+        )
+    assert "IMP-46" in str(exc.value)
+
+
+def test_save_rejects_empty_key():
+    with pytest.raises(ValueError):
+        save_proposal(
+            "", _proposal(), visual_check_passed=True, user_approved=True
+        )
+
+
+def test_save_rejects_non_proposal_object():
+    with pytest.raises(TypeError):
+        save_proposal(
+            "k",
+            {"proposal_kind": "builder_options_patch"},  # type: ignore[arg-type]
+            visual_check_passed=True,
+            user_approved=True,
+        )
+
+
+def test_gate_error_is_not_notimplementederror():
+    with pytest.raises(AiFallbackCacheGateError):
+        save_proposal(
+            "k", _proposal(), visual_check_passed=False, user_approved=True
+        )
+    assert not issubclass(AiFallbackCacheGateError, NotImplementedError)