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>

tests/test_v4_fallback_policy_loader.py

@@ -0,0 +1,109 @@
+"""IMP-38 U1 — v4_fallback_policy.yaml loader test.
+
+Verify:
+  - load_v4_fallback_policy() returns dict with expected keys
+  - yaml parsed correctly (usable_threshold, default_max_rank, extended_max_rank, policy_type)
+  - graceful fallback when yaml missing → _V4_FALLBACK_POLICY_DEFAULT
+  - _V4_FALLBACK_POLICY_CACHE pattern (lazy load, mirror of _CATALOG_CACHE)
+  - load_frame_contracts() shape unchanged (separate yaml, catalog 오염 X)
+
+4 round 합의 (#67):
+  - Codex #1: separate yaml (not frame_contracts.yaml top-level)
+  - Codex #3: load_frame_contracts() shape 변경 X
+"""
+from __future__ import annotations
+
+import importlib
+from pathlib import Path
+from unittest.mock import patch
+
+import pytest
+
+
+PROJECT_ROOT = Path(__file__).parent.parent
+V4_POLICY_PATH = PROJECT_ROOT / "templates" / "phase_z2" / "catalog" / "v4_fallback_policy.yaml"
+CATALOG_PATH = PROJECT_ROOT / "templates" / "phase_z2" / "catalog" / "frame_contracts.yaml"
+
+
+def _reset_caches():
+    """Reset module-level caches for test isolation."""
+    import src.phase_z2_mapper as mapper
+    mapper._V4_FALLBACK_POLICY_CACHE = None
+    mapper._CATALOG_CACHE = None
+
+
+@pytest.fixture(autouse=True)
+def clean_caches():
+    _reset_caches()
+    yield
+    _reset_caches()
+
+
+def test_v4_fallback_policy_yaml_exists():
+    """IMP-38 U1 — separate yaml file must exist."""
+    assert V4_POLICY_PATH.exists(), (
+        f"v4_fallback_policy.yaml not found at {V4_POLICY_PATH}. "
+        "IMP-38 U1 expects separate yaml (Codex #1 corr — not frame_contracts.yaml top-level)."
+    )
+
+
+def test_load_v4_fallback_policy_returns_dict_with_expected_keys():
+    """load_v4_fallback_policy() must return dict with policy keys."""
+    from src.phase_z2_mapper import load_v4_fallback_policy
+    policy = load_v4_fallback_policy()
+    assert isinstance(policy, dict)
+    expected_keys = {"policy_type", "usable_threshold", "default_max_rank", "extended_max_rank"}
+    missing = expected_keys - set(policy.keys())
+    assert not missing, f"missing keys in v4_fallback_policy: {missing}"
+
+
+def test_load_v4_fallback_policy_values_match_yaml():
+    """Loaded policy values must match v4_fallback_policy.yaml (initial commit)."""
+    from src.phase_z2_mapper import load_v4_fallback_policy
+    policy = load_v4_fallback_policy()
+    assert policy["policy_type"] == "dynamic_usable_count_based"
+    assert policy["usable_threshold"] == 1
+    assert policy["default_max_rank"] == 3
+    assert policy["extended_max_rank"] == 32
+
+
+def test_load_v4_fallback_policy_cache_pattern():
+    """_V4_FALLBACK_POLICY_CACHE pattern — second call returns same dict (lazy load)."""
+    from src.phase_z2_mapper import load_v4_fallback_policy
+    policy_a = load_v4_fallback_policy()
+    policy_b = load_v4_fallback_policy()
+    assert policy_a is policy_b, "cache pattern violated (should return same dict instance)"
+
+
+def test_load_v4_fallback_policy_graceful_when_yaml_missing():
+    """yaml 파일 없을 시 → _V4_FALLBACK_POLICY_DEFAULT (extended_max_rank=3, byte-identical pre-IMP-38)."""
+    import src.phase_z2_mapper as mapper
+    with patch.object(mapper, "V4_FALLBACK_POLICY_PATH", PROJECT_ROOT / "tests" / "__nonexistent_policy.yaml"):
+        # reset cache to force reload via patched path
+        mapper._V4_FALLBACK_POLICY_CACHE = None
+        policy = mapper.load_v4_fallback_policy()
+        assert policy["default_max_rank"] == 3
+        assert policy["extended_max_rank"] == 3, (
+            "graceful fallback must keep extended==default (byte-identical pre-IMP-38)"
+        )
+
+
+def test_load_frame_contracts_shape_unchanged():
+    """Codex #3 LOCK — load_frame_contracts() must still return template_id → entry dict."""
+    from src.phase_z2_mapper import load_frame_contracts, load_v4_fallback_policy
+    catalog = load_frame_contracts()
+    policy = load_v4_fallback_policy()
+
+    # catalog 의 key 가 모두 frame entry (dict with template_id/frame_id) 여야 함
+    for key, entry in catalog.items():
+        assert isinstance(entry, dict), f"catalog entry {key} should be dict"
+        assert "template_id" in entry, f"catalog entry {key} missing template_id (policy bleed?)"
+
+    # policy keys 는 catalog 에 안 들어감
+    policy_keys = {"policy_type", "usable_threshold", "default_max_rank", "extended_max_rank"}
+    catalog_top_keys = set(catalog.keys())
+    bleed = policy_keys & catalog_top_keys
+    assert not bleed, (
+        f"policy keys leaked into frame_contracts.yaml: {bleed}. "
+        "Codex #1 corr violated — policy must stay in separate v4_fallback_policy.yaml."
+    )