feat(#85): IMP catalog builder invariant + VP runtime gate (u1~u7)

- u1: BuilderMissingError(FitError) — narrow exception aligned with pipeline catch
- u2: load_frame_contracts catalog invariant + VP skip + CatalogInvariantError
- u3a: audit CLI I1~I3 (partial existence / declared builder / registry membership)
- u3b: audit CLI I4 (slot_payload refs vs declared/generated payload keys)
- u4: lookup_v4_candidates VP filter (lookup_v4_all_judgments raw telemetry untouched)
- u5: catalog invariant regression coverage + temp non-VP failure fixtures
- u6: mdx04 VP routing fixture tests (sw_dependency_four_problems excluded from live)
- u7: tests/conftest.py env isolation + mdx03/mdx04/mdx05 subprocess smoke

Targeted 74 PASS (12.31s). Full regression 1063 PASS (87.70s). Audit CLI clean.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

tests/test_phase_z2_mapper_builder_missing.py

@@ -0,0 +1,85 @@
+"""IMP-#85 u1 — mapper missing-builder dispatch raises BuilderMissingError.
+
+Scope (Stage 2 lock):
+  - `BuilderMissingError` exists and is a subclass of `FitError`.
+  - `map_with_contract` raises `BuilderMissingError` when
+    `contract.payload.builder` references an unknown registry entry, OR
+    when `payload.builder` is empty/missing.
+  - Because it subclasses `FitError`, the existing pipeline
+    `except FitError` route in `src/phase_z2_pipeline.py` continues to
+    catch the failure and emit an `adapter_needed` record instead of a
+    hard crash (mdx04 `sw_dependency_four_problems` / `cards_4_grid`
+    regression evidence).
+"""
+from __future__ import annotations
+
+from types import SimpleNamespace
+
+import pytest
+
+from src.phase_z2_mapper import (
+    BuilderMissingError,
+    FitError,
+    PAYLOAD_BUILDERS,
+    map_with_contract,
+)
+
+
+def _make_section(raw_content: str = "- a\n- b\n- c"):
+    return SimpleNamespace(
+        section_id="test-sec",
+        raw_content=raw_content,
+        title="t",
+        order=1,
+    )
+
+
+def test_builder_missing_error_is_fit_error_subclass():
+    assert issubclass(BuilderMissingError, FitError)
+
+
+def test_unknown_builder_raises_builder_missing_error():
+    unknown = "definitely_not_a_registered_builder"
+    assert unknown not in PAYLOAD_BUILDERS
+    contract = {
+        "template_id": "fake_contract_unknown_builder",
+        "source_shape": "top_bullets",
+        "cardinality": {},
+        "payload": {"builder": unknown},
+    }
+    with pytest.raises(BuilderMissingError) as exc:
+        map_with_contract(_make_section(), contract)
+    assert unknown in str(exc.value)
+    assert "fake_contract_unknown_builder" in str(exc.value)
+
+
+def test_missing_builder_field_raises_builder_missing_error():
+    contract = {
+        "template_id": "fake_contract_missing_builder_field",
+        "source_shape": "top_bullets",
+        "cardinality": {},
+        "payload": {},
+    }
+    with pytest.raises(BuilderMissingError) as exc:
+        map_with_contract(_make_section(), contract)
+    assert "missing payload.builder" in str(exc.value)
+
+
+def test_builder_missing_error_caught_by_fit_error_handler():
+    """Pipeline 의 `except FitError` 경로가 그대로 잡아주는지 검증.
+
+    실제 pipeline import 없이 동일 패턴을 재현하여 subclass 의 의도된
+    routing 효과(adapter_needed) 가 깨지지 않는지 확인.
+    """
+    contract = {
+        "template_id": "fake_contract_routing_check",
+        "source_shape": "top_bullets",
+        "cardinality": {},
+        "payload": {"builder": "no_such_builder"},
+    }
+    caught = False
+    try:
+        map_with_contract(_make_section(), contract)
+    except FitError:
+        caught = True
+    assert caught, "BuilderMissingError must propagate through `except FitError`"