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>

src/phase_z2_mapper.py

@@ -579,12 +579,23 @@ def _build_compare_table_2col(section, units, contract) -> dict:
 
     builder_options :
       item_parser                  : ITEM_PARSERS key (예: `compare_row_2col_item`)
-      col_a_label_default          : col_a header (MDX 미명시 시 fallback. F1-a fix)
-      col_b_label_default          : col_b header (MDX 미명시 시 fallback)
+      col_a_label_default          : col_a header literal in catalog.
+                                     Semantics depend on col_a_label_default_role.
+      col_a_label_default_role     : "placeholder" | "fallback" (IMP-40 #69).
+                                     placeholder = Figma visual placeholder; suppressed
+                                       at runtime → col_a_label emitted as "".
+                                     fallback    = MDX 미명시 시 catalog literal 사용.
+                                     absent      = legacy contracts default to fallback.
+      col_b_label_default          : col_b header literal (same policy as col_a).
+      col_b_label_default_role     : same role discriminator for col_b (IMP-40 #69).
       strip_col_prefix_aliases     : list[str] — col_a/col_b 값의 prefix `<alias>:`
                                      를 strip (Codex round 43 §F1-b — narrow alias).
                                      예 : ["BIM", "DX"]. default [] (no stripping).
       max_rows                     : N (default 999 — practical 한계).
+
+    NOTE: MDX 측 col_a_label / col_b_label inflow 경로 없음
+    (compare_row_2col_item parser → {label,col_a,col_b}, _resolve_title → title only).
+    placeholder role 은 col_*_label 을 빈 문자열로 확정 — 정책 결정점은 catalog 한 곳뿐.
     """
     options = contract["payload"]["builder_options"]
     parser_name = options["item_parser"]
@@ -595,8 +606,21 @@ def _build_compare_table_2col(section, units, contract) -> dict:
             f"but ITEM_PARSERS has no such entry."
         )
 
-    col_a_label = options.get("col_a_label_default", "")
-    col_b_label = options.get("col_b_label_default", "")
+    def _resolve_label_default(col_key: str) -> str:
+        default_key = f"{col_key}_label_default"
+        role_key = f"{col_key}_label_default_role"
+        role = options.get(role_key, "fallback")
+        if role == "placeholder":
+            return ""
+        if role == "fallback":
+            return options.get(default_key, "")
+        raise ValueError(
+            f"Contract '{contract['template_id']}' builder_options.{role_key}='{role}' "
+            f"is invalid; expected 'placeholder' or 'fallback' (IMP-40 #69)."
+        )
+
+    col_a_label = _resolve_label_default("col_a")
+    col_b_label = _resolve_label_default("col_b")
     strip_aliases = options.get("strip_col_prefix_aliases", []) or []
     max_rows = options.get("max_rows", 999)