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/prompts.py

@@ -0,0 +1,80 @@
+"""IMP-33 u3 — AI fallback prompt builder (fallback path only).
+
+System+user prompt for the Anthropic client (u4). MDX is READ-ONLY
+(`feedback_ai_isolation_contract`); output is constrained to the u2
+schema; frame_id swap is forbidden (V4 rank-1 protected,
+`feedback_phase_z_spacing_direction`). Inputs per Stage 2 plan: V4
+result (route=ai_adaptation_required, cardinality), frame_contract,
+frame_visual HTML, figma_to_html_agent partial JSON, Internal Region,
+MDX text.
+"""
+from __future__ import annotations
+
+import json
+from typing import Any
+
+from src.phase_z2_ai_fallback.schema import FORBIDDEN_KINDS, ProposalKind
+
+V4_ROUTE_AI_ADAPTATION = "ai_adaptation_required"
+
+_ALLOWED_KINDS = ", ".join(sorted(k.value for k in ProposalKind))
+_FORBIDDEN_KINDS = ", ".join(sorted(FORBIDDEN_KINDS))
+
+SYSTEM_PROMPT = (
+    "You are an IMP-33 AI fallback adapter for Phase Z slide composition.\n"
+    "STRICT RULES:\n"
+    "  1. MDX text in the user payload is READ-ONLY. Do NOT rewrite, "
+    "compress, or paraphrase MDX.\n"
+    "  2. Output MUST be a single JSON object conforming to AiFallbackProposal.\n"
+    f"  3. proposal_kind MUST be one of: {_ALLOWED_KINDS}.\n"
+    f"  4. Do NOT propose any of: {_FORBIDDEN_KINDS}.\n"
+    "  5. Do NOT change frame_id — V4 rank-1 frame is locked.\n"
+    "  6. Keep declared frame slots (text/table/image/details) populated.\n"
+    "  7. Respect Internal Region containment; place content units within "
+    "the declared region only."
+)
+
+
+def build_ai_fallback_prompt(
+    *,
+    v4_result: dict[str, Any],
+    frame_contract: dict[str, Any],
+    frame_visual_html: str,
+    figma_partial_json: dict[str, Any],
+    internal_region: dict[str, Any],
+    mdx_text: str,
+) -> dict[str, str]:
+    """Build system+user prompt strings for the fallback AI adapter.
+
+    Raises:
+        ValueError: when ``v4_result.route`` is not
+            ``ai_adaptation_required`` — the fallback prompt MUST NOT be
+            built outside this route (normal-path AI call count must
+            remain 0; PZ-1).
+    """
+    route = v4_result.get("route") or v4_result.get("imp05_route_hint")
+    if route != V4_ROUTE_AI_ADAPTATION:
+        raise ValueError(
+            f"build_ai_fallback_prompt: v4_result.route={route!r} is not "
+            f"{V4_ROUTE_AI_ADAPTATION!r}; fallback prompt MUST NOT be built "
+            "outside the AI adaptation route."
+        )
+    user_payload = {
+        "v4": {
+            "route": route,
+            "cardinality": v4_result.get("cardinality")
+            or v4_result.get("cardinality_signature"),
+            "label": v4_result.get("label"),
+            "frame_id": v4_result.get("frame_id"),
+            "rank": v4_result.get("rank"),
+        },
+        "frame_contract": frame_contract,
+        "frame_visual_html": frame_visual_html,
+        "figma_partial_json": figma_partial_json,
+        "internal_region": internal_region,
+        "mdx_text_READ_ONLY": mdx_text,
+    }
+    return {
+        "system": SYSTEM_PROMPT,
+        "user": json.dumps(user_payload, ensure_ascii=False),
+    }