feat(rebar): 정면 dimension chains computed from the 3D bar positions (기둥 중심 = 0)

- chainDims groups the repeated gaps (n@pitch = total) and writes a single gap as its length.
- Top chain: the 전단·띠 stations (C9 @250, C10 @300) from the coping ends.
- Lower chain: the upper C14 bars measured from the column centre (0), so
  the 10@1000 span starts at the column centre.
- Stations are the lowest s of their pieces; pieces within 160 mm are one station.
- P1 gives the 시안 values: 300 · 18@250=4500 · 8@300=2400 · 18@250=4500 · 300 and
  900 · 10@1000=10000 · 900.

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

src/main.ts

@@ -1092,7 +1092,7 @@ function renderCopingDrawings(box: HTMLDivElement, pier: Pier): void {
   wrap.appendChild(row);
   const fig = el('figure', undefined, 'ground-fig');
   if (copingView === 'front') {
-    fig.innerHTML = copingFrontSvg(rebar3d.records, L * 1000);
+    fig.innerHTML = copingFrontSvg(rebar3d.records, L * 1000, pier.model.columns[0]?.s ?? 0);
     fig.appendChild(el('figcaption', '정면 · 교축 방향에서 본 코핑 배근 (3D 철근과 같은 데이터)', 'muted small'));
   } else {
     const at = copingStation ?? L / 2;