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>

docs/architecture/IMP-17-CARVE-OUT.md

@@ -1,6 +1,6 @@
 # IMP-17 — AI repair fallback infrastructure (carve-out)
 
-**Status**: carve-out, **design-only**. Normal-path AI calls = 0. No runtime fallback code lands until the activation gate clears.
+**Status**: carve-out infra **scaffolded under IMP-33** (issue #61, Stage 3 u1~u11). Normal-path AI calls = 0 (PZ-1) — `ai_fallback_enabled` flag default `False` in `src/config.py`. Runtime AI is reachable only via fallback path entry points; Step 12 entry is provisional-gated, Step 17 entry is structurally blocked behind IMP-34 + IMP-35.
 
 **Source anchors**
 - IMP-17 backlog row — [`PHASE-Z-IMPLEMENTATION-ISSUE-BACKLOG.md`](PHASE-Z-IMPLEMENTATION-ISSUE-BACKLOG.md):68 (carve-out — normal path 밖, soft link IMP-04 + IMP-05).
@@ -42,3 +42,14 @@ Phase Q `content_editor.py` 는 **Archive Candidate** ([`PHASE-Q-AUDIT.md`](PHAS
 - AI 호출은 normal path 에 없다 (Phase Z 원칙, [memory `feedback_ai_isolation_contract`](../../README.md)).
 - 출력 단위는 항상 content_object / Internal Region / Frame Slot 또는 restructuring proposal — HTML 구조 / 레이아웃 / 프리셋 결정 X.
 - Phase Q 자산 (Kei persona prompts, Kei-API endpoint, persona retry semantics) 과 단절. Phase Z 의 fallback runtime 은 별도 prompt / endpoint 설계로 출발한다 (본 carve-out 활성 시).
+
+## Runtime module surface (IMP-33 u1~u11 binding)
+
+| Axis | Binding |
+|---|---|
+| Module path | `src/phase_z2_ai_fallback/` (locked by [`IMP-31-GATE-AUDIT.md`](IMP-31-GATE-AUDIT.md):31,50,56). |
+| Step 12 entry | `src.phase_z2_ai_fallback.step12.gather_step12_ai_repair_proposals` — IMP-30 provisional gate (`not_provisional` skip) AND reject gate (`design_reference_only_no_ai` skip) AND non-AI route catch-all run BEFORE `route_ai_fallback`. |
+| Step 17 entry | `src.phase_z2_ai_fallback.step17.gather_step17_ai_repair_proposals` — STRUCTURALLY BLOCKED. Every unit returns `skip_reason="step17_ai_blocked_imp_34_35_prerequisites_missing"`. Module does NOT import `route_ai_fallback` / `AiFallbackClient` / `anthropic`. |
+| Cascade order | `src.phase_z2_ai_fallback.step17.OVERFLOW_CASCADE_ORDER = (DETERMINISTIC, POPUP, AI_REPAIR, USER_OVERRIDE)` — single source of truth for Step 17 consumers. Aligns with line 16 of this doc. |
+| IMP-46 cache gate | `src.phase_z2_ai_fallback.cache.save_proposal(..., visual_check_passed, user_approved)` raises `AiFallbackCacheGateError` unless BOTH gates are True; storage backend then raises `NotImplementedError` (IMP-46 marker). `read_proposal` returns `None` until IMP-46 lands a backend. |
+| AST isolation | `tests/phase_z2_ai_fallback/test_ast_isolation.py` parses every `*.py` under `src/phase_z2_ai_fallback/` and forbids Phase Q runtime / Kei client / `src.phase_z2_*` (non-fallback) imports. Whitelist = `src.config` + intra-package + stdlib + `anthropic` + `pydantic`. |