feat(#67): IMP-38 V4 max_rank policy formalization (u1~u3, 4 round consensus)

- u1: separate templates/phase_z2/catalog/v4_fallback_policy.yaml + load_v4_fallback_policy() loader
  (catalog pollution prevention — Codex #1 correction)
- u2: dynamic effective max_rank in lookup_v4_match_with_fallback (3-variable ceiling min,
  Codex #2 correction: min(configured, len(judgments_full32))) + 3-tier usable predicate
  (status + catalog + optional capacity) + trace 8 fields (requested/default/configured_extended/
  judgments_count/effective_extended_ceiling/effective_max_rank/usable_count/policy_applied)
- u3: 2 production call site cleanup (max_rank=3 removed, HEAD baseline) + tracked
  Front/vite.config.ts PHASE_Z_MAX_RANK env retired + 4 regression scenarios

verified: 32 passed (IMP-38 focused scope) — IMP-05 L4 dedup / L2 schema preserved,
IMP-30 allow_provisional byte-identical, caller_override backward compat (tests)

Stage cycle (#67, 7 round Claude + 5 round Codex):
- Stage 1: Claude #1 -> Codex #1 YES + 5 corrections
- Stage 2 r1+r2: Claude #2-#4 -> Codex #2 Q2 -> Codex #3 YES (4 round consensus LOCK 23195)
- Stage 3 U1+U2+U3: Claude #5-#9 -> Codex #6 NO 4to3 correction -> Codex #7 YES -> Codex #8 YES
- Stage 4: Claude #11 -> Codex #9 (anchor attribution nuance) -> Codex #10 readiness -> Codex #11

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

src/phase_z2_mapper.py

@@ -32,6 +32,7 @@ import yaml
 
 PROJECT_ROOT = Path(__file__).parent.parent
 CATALOG_PATH = PROJECT_ROOT / "templates" / "phase_z2" / "catalog" / "frame_contracts.yaml"
+V4_FALLBACK_POLICY_PATH = PROJECT_ROOT / "templates" / "phase_z2" / "catalog" / "v4_fallback_policy.yaml"
 
 
 class FitError(Exception):
@@ -57,6 +58,44 @@ def get_contract(template_id: str) -> dict | None:
     return load_frame_contracts().get(template_id)
 
 
+# ─── V4 fallback policy loading (IMP-38) ──────────────────────────
+
+_V4_FALLBACK_POLICY_CACHE: dict | None = None
+
+_V4_FALLBACK_POLICY_DEFAULT: dict = {
+    "policy_type": "static",
+    "usable_threshold": 1,
+    "default_max_rank": 3,
+    "extended_max_rank": 3,  # graceful: yaml 없을 시 확장 X (byte-identical to pre-IMP-38)
+}
+
+
+def load_v4_fallback_policy() -> dict:
+    """IMP-38 V4 fallback policy loader (separate yaml, catalog 오염 방지).
+
+    Returns dict with keys: policy_type, usable_threshold, default_max_rank, extended_max_rank.
+
+    Codex #1 권장: frame_contracts.yaml top-level 오염 회피 (별 yaml).
+    Codex #3 LOCK: load_frame_contracts() shape 변경 X (이 함수는 별 cache).
+
+    Graceful fallback:
+        yaml 파일 없을 시 → _V4_FALLBACK_POLICY_DEFAULT (default_max_rank=3, extended=3)
+        → backward compat byte-identical to pre-IMP-38 behavior.
+
+    Returns:
+        dict — 정책 키 (정책 yaml 의 superset 가능, 알 수 없는 키는 무시 권장).
+    """
+    global _V4_FALLBACK_POLICY_CACHE
+    if _V4_FALLBACK_POLICY_CACHE is None:
+        if V4_FALLBACK_POLICY_PATH.exists():
+            loaded = yaml.safe_load(V4_FALLBACK_POLICY_PATH.read_text(encoding="utf-8")) or {}
+            # merge with default (yaml 키 부분 누락 시 default 로 fall through)
+            _V4_FALLBACK_POLICY_CACHE = {**_V4_FALLBACK_POLICY_DEFAULT, **loaded}
+        else:
+            _V4_FALLBACK_POLICY_CACHE = dict(_V4_FALLBACK_POLICY_DEFAULT)
+    return _V4_FALLBACK_POLICY_CACHE
+
+
 # ─── Source-shape splitters ──────────────────────────────────────
 
 def _split_top_bullets(content: str) -> list[tuple[str, list[str]]]: