feat(#69): IMP-40 u1~u6 frame contract label_default placeholder/fallback role discriminator (BIM/DX leak fix)

- catalog (frame_contracts.yaml): F18 bim_dx_comparison_table col_a/col_b
  label_default_role=placeholder; F30 industry_current_status_three_col +
  F31 industry_characteristics_three_col col_a/col_b/col_c forward-compat
  placeholder; F33 engn_sw_three_types untouched (no label_default).
- mapper (_build_compare_table_2col): generic _resolve_label_default(col_key)
  branches on <col>_label_default_role — placeholder -> '' (Figma placeholder
  suppressed at runtime), fallback -> catalog literal (legacy default), unknown
  -> ValueError with template_id + role_key + value. Absent role defaults to
  fallback (backward compat for contracts without discriminator).
- tests (tests/phase_z2/test_imp40_label_default_role.py): u4 generic matrix
  (placeholder / fallback / absent / unknown / 3-col axis) + u5 F18-reuse
  non-BIM/DX synthetic rows asserting placeholder labels emit '' and BIM/DX
  literal tokens do not leak.
- snapshot (tests/integration/__snapshots__/slot_payload.json): mdx 01 F18
  string_slot_nonempty.col_a_label/col_b_label True -> False (u6 expected
  drift from u3 placeholder -> empty string flip). slot_names + rows + title
  preserved.

Verification:
- imp40_label_default_role: 6/6 PASSED
- phase_z2 sweep: 608/608 PASSED
- multi_mdx_regression: 50/50 PASSED
- cross-suite sweep: 662/662 PASSED
- BIM/DX literal grep on mapper + new test: 0 hits
- No mdx-specific branches (mdx 03/04/05 grep on mapper: 0 hits)

Guardrails: no MDX 03/04/05 hardcoding (catalog policy only); no spacing
shrink; no auto frame swap on reject; no AI call at Step 12; F33 untouched.

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

tests/integration/__snapshots__/slot_payload.json

@@ -8,7 +8,7 @@
       "slot_names": ["col_a_label", "col_b_label", "rows", "title"],
       "list_slot_counts": {"rows": 2},
       "dict_slot_sub_counts": {},
-      "string_slot_nonempty": {"col_a_label": true, "col_b_label": true, "title": true}
+      "string_slot_nonempty": {"col_a_label": false, "col_b_label": false, "title": true}
     },
     {
       "position": "bottom",