feat(rebar): second dimension line and legend on the coping 정면 (시안 ③)

- The column-centre line 900 · 10@1000=10000 (기둥 중심부터) · 900, from the
  design values, under the first dimension.
- A legend of the bar groups and their colours under the drawing.

Co-Authored-By: Claude Haiku 5.5 <noreply@anthropic.com>

tests/coping-rebar-svg.test.ts

@@ -55,3 +55,12 @@ describe('정면 번호 풍선과 치수 (승인 2026-10-26, 시안 ③)', () =>
     expect(svg).toContain('코핑 길이 12000');
   });
 });
+
+describe('정면 두 번째 치수선과 범례 (승인 2026-10-26, 시안)', () => {
+  const svg = copingFrontSvg(scene.records, model.copingL * 1000);
+  it('writes the column-centre span 10@1000=10000 and the legend', () => {
+    expect(svg).toContain('10@1000=10000 (기둥 중심부터)');
+    expect(svg).toContain('주철근 (C1–C8)');
+    expect(svg).toContain('전단·띠 (C9–C11)');
+  });
+});