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/phase_z2/fixtures/catalog/missing_builder_non_vp.yaml

@@ -0,0 +1,17 @@
+# IMP-#85 u5 fixture — non-VP contract whose payload.builder is absent from
+# `PAYLOAD_BUILDERS`. Drives the u2 boot invariant + audit I3 negative paths.
+#
+# Scope (Stage 2 lock): regression coverage only. Not a runtime catalog entry.
+# Frame id is in the 9999xxx range so any accidental cross-reference is obvious.
+
+imp85_u5_missing_builder_frame:
+  template_id: imp85_u5_missing_builder_frame
+  frame_id: 9999001
+  family: imp85_u5_fixture
+  source_shape: top_bullets
+  cardinality:
+    strict: 3
+  payload:
+    title:
+      source: section.title
+    builder: definitely_not_a_registered_builder_imp85_u5

tests/phase_z2/fixtures/catalog/undeclared_slot_ref_non_vp.yaml

@@ -0,0 +1,23 @@
+# IMP-#85 u5 fixture — non-VP contract whose `items_with_role` builder produces
+# a `slot_payload.<array_root>` key the partial never references. Drives the
+# audit I4 (generated-key-orphan) negative path.
+#
+# Scope (Stage 2 lock): regression coverage only. The corresponding partial is
+# written into a tmp dir by the test (it must NOT use `slot_payload[...]`
+# bracket access, otherwise I4 suppresses correctly and the assertion fails).
+
+imp85_u5_undeclared_slot_frame:
+  template_id: imp85_u5_undeclared_slot_frame
+  frame_id: 9999002
+  family: imp85_u5_fixture
+  source_shape: top_bullets
+  cardinality:
+    strict: 3
+  payload:
+    title:
+      source: section.title
+    builder: items_with_role
+    builder_options:
+      item_parser: pillar_item
+      array_root: orphan_array_root_imp85_u5
+      role_field: color_class