fix(IMP-04): F18 F1 follow-ups — defaults + narrow alias + cardinality clarify

Same-frame F1 follow-up per Codex round 43 (#15527). matrix §4.1 Fix 7
4-class F1 path (no Track A pause, small fixes + Codex re-review).

Three fixes :

1. F1-a — explicit col_a/col_b label defaults
   - Previous (c7b0f5b) used empty defaults `col_a_label_default: ""` /
     `col_b_label_default: ""`, so an upstream MDX path without explicit
     column headers would render blank header cells.
   - Fix : set `col_a_label_default: "BIM"` and `col_b_label_default: "DX"`
     in F18 contract. Frame intent is the BIM-vs-DX comparison, so the
     headers are semantic and must not silently become blank.

2. F1-b — narrow prefix-stripping aliases (parser → builder option)
   - Previous parser used a broad regex
     `^[A-Za-z가-힣]{1,8}\s*:\s*(.+)$` to strip any short prefix before
     `:`. That could accidentally remove meaningful Korean/English
     prefixes from real cell content.
   - Fix : remove auto-stripping from `parse_compare_row_2col_item`.
     Stripping is now configurable via builder option
     `strip_col_prefix_aliases: [<list of exact aliases>]` and applied
     by `_build_compare_table_2col`. F18 contract uses `["BIM", "DX"]`,
     so only `BIM:` / `DX:` (with optional fullwidth `：`) prefixes are
     stripped; other Korean/English colons in real content stay intact.
   - Parser signature unchanged. Builder is the single place that owns
     the stripping policy.

3. F1-c — cardinality semantic clarification
   - Previous top-level `cardinality.strict: 2` was ambiguous: it could
     be read as `row strict: 2`. Rows are actually `1..12` via
     `sub_zones.rows.cardinality`.
   - Fix : add YAML comment that the top-level strict 2 = column count
     (col_a / col_b), not row count. Per-sub_zone cardinality remains
     authoritative for rows.

Verification :
- python -m py_compile src/phase_z2_mapper.py : PASS
- python scripts/smoke_frame_render.py --self-check : PASS 7/7 (F18
  fixture rendered unchanged at 4211 chars; smoke harness only loaded
  the partial, builder/parser logic not directly exercised in smoke)
- Manual builder/parser invocation test with synthetic units :
  - col_a_label / col_b_label resolve to "BIM" / "DX" defaults.
  - `BIM: Only 3D` → `Only 3D` (alias-stripped).
  - `DX: BIM << DX (ENG. 포함)` → `BIM << DX (ENG. 포함)` (alias-stripped).
  - `분야별 단절` → `분야별 단절` (no BIM/DX prefix, untouched).
  - This matches the F1-b narrow-alias intent.
- python scripts/smoke_frame_render.py bim_dx_comparison_table
  --render-to data/runs/imp04_f18_visual_r2 : PASS, R3 artifact written
  with same character count, generic viewer title.

scope-lock honored : no V4 logic, no new builder/parser added (only
behavior refinement of existing F18 builder/parser/contract), no other
partial, no Phase Z production render, no Phase R'/AI/Kei changes.

4-class status (F18 post-F1) :
- class 1 readiness : adapter cleanup complete — defaults explicit,
  aliases narrow, cardinality semantically clear.
- class 2 content-fit : still a watch item (long Korean wrapping,
  6+ rows). max_rows=12 protection unchanged.
- class 3 / 4 : N/A.

Refs Gitea #4 (IMP-04 Track A frame 4 — F1 follow-up per Codex round 43)

templates/phase_z2/catalog/frame_contracts.yaml

@@ -398,8 +398,10 @@ bim_dx_comparison_table:
   family: table
 
   source_shape: top_bullets
+  # NOTE (Codex round 43 §F1-c) : top-level `cardinality.strict: 2` = *column 수*
+  # (col_a / col_b). data row 수 는 별 — `sub_zones.rows.cardinality` 의 `{min:1, max:12}`.
   cardinality:
-    strict: 2                              # 2 columns (BIM vs DX 등)
+    strict: 2                              # 2 columns (col_a / col_b) — NOT row count
     overflow_policy: abort_or_review
 
   role_order:
@@ -443,6 +445,9 @@ bim_dx_comparison_table:
     builder: compare_table_2col
     builder_options:
       item_parser: compare_row_2col_item    # NEW parser — top_bullet → {label, col_a, col_b}
-      col_a_label_default: ""               # MDX 명시 또는 frame default
-      col_b_label_default: ""
+      col_a_label_default: "BIM"            # F1-a (Codex round 43) — explicit default
+      col_b_label_default: "DX"             # F1-a — explicit default
+      strip_col_prefix_aliases:             # F1-b (Codex round 43) — narrow alias 만 strip
+        - "BIM"
+        - "DX"
       max_rows: 12                           # typical 4-8, overflow 보호