feat(rebar): C15·C16 폭 간격을 2 × 주철근 기본간격에 연동

- stairBars가 pitch를 받는다 (기본 2 × 기본간격 = 250, P1 그대로)
- 3D와 철근표가 같은 pitch를 쓴다
- 철근표 간격·실간격: 2 × 기본간격, 근거는 기본값/입력

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

tests/rebar-coping-main.test.ts

@@ -40,12 +40,16 @@ describe('C1–C8 상·하면 주철근 (코핑철근.md §5, plan C3)', () => {
     expect([c1.spacing, c1.actual, c1.basis]).toEqual(['250', '125', 'default']);
     const c12 = base.find((r) => r.name === 'C12')!;
     expect(c12.basis).toBe('default');
+    const c15 = base.find((r) => r.name === 'C15')!;
+    expect([c15.spacing, c15.actual, c15.basis]).toEqual(['250', '250', 'default']);
 
     const m = buildPier({ ...loadP1(), copingRebar: { mainSpacing: 140 }, blockRebar: { spacingS1: 130 } });
     const rows = rebarSummary(m).rows;
     const c8 = rows.find((r) => r.name === 'C8')!;
     expect([c8.spacing, c8.actual, c8.basis]).toEqual(['280', '140', 'input']);
     expect(rows.find((r) => r.name === 'C12')!.basis).toBe('default');
+    const c16 = rows.find((r) => r.name === 'C16')!;
+    expect([c16.spacing, c16.actual, c16.basis]).toEqual(['280', '280', 'input']);
     const s1 = rows.find((r) => r.name === 'S1')!;
     expect([s1.spacing, s1.actual, s1.basis]).toEqual(['130', '130', 'input']);
   });