fix(rebar): cut the C–C section through the selected pair only

- The section was drawing every bearing on the same 교축 line, so the blocks and
  dimension text stacked on top of each other. It now cuts through the selected
  pair (①·⑥ …), which the ③ card also shows as chips, shared with ②.

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

src/main.ts

@@ -1017,6 +1017,20 @@ function renderBlockRebarSettings(box: HTMLDivElement, pier: Pier): void {
   const wrap = el('div', undefined, 'bearing-ref');
   const current: BlockRebarSettings = { ...DEFAULT_BLOCK_REBAR, ...pier.draft.blockRebar };
   wrap.appendChild(el('p', '받침블럭 배근 설정 (기본값 = P1 · 범위는 칸 옆에 표시)', 'lbl'));
+  // 단면 쌍 (the same selection as ②): the C–C section cuts through one pair.
+  const pairList = pairEccentricities(pier.model.blocks);
+  const chipRow = el('div', undefined, 'opts');
+  pairList.forEach((p, i) => {
+    const chip = el('button', `${PAIR_CHIP_LABELS[p.frontNo - 1] ?? p.frontNo}·${PAIR_CHIP_LABELS[p.backNo - 1] ?? p.backNo}`, 'btn small');
+    chip.type = 'button';
+    chip.setAttribute('aria-pressed', String(i === bearingPairIndex));
+    chip.addEventListener('click', () => {
+      bearingPairIndex = i;
+      renderAll();
+    });
+    chipRow.appendChild(chip);
+  });
+  wrap.appendChild(chipRow);
   const grid = el('div', undefined, 'edit-fields');
   for (const f of BLOCK_REBAR_FIELDS) {
     const row = el('div', undefined, 'edit-row');
@@ -1034,8 +1048,8 @@ function renderBlockRebarSettings(box: HTMLDivElement, pier: Pier): void {
   const n = pier.model.integratedBlocks.length;
   wrap.appendChild(el('p', `적용 블럭: ${n > 0 ? `일체 ${n}쌍` : `분리 ${pier.model.blocks.length}개`} (전체)`, 'muted small'));
   const fig = el('figure', undefined, 'ground-fig');
-  fig.innerHTML = blockRebarSectionSvg(pier.model);
-  fig.appendChild(el('figcaption', '단면 C–C · 교축 방향 · S2 점(상면), S1 선과 다리(⊓) · 치수 mm', 'muted small'));
+  fig.innerHTML = blockRebarSectionSvg(pier.model, Math.min(bearingPairIndex, Math.max(0, pairEccentricities(pier.model.blocks).length - 1)));
+  fig.appendChild(el('figcaption', '단면 C–C · 교축 방향 · 선택한 쌍 · S2 점(상면), S1 선과 다리(⊓) · 치수 mm', 'muted small'));
   wrap.appendChild(fig);
   box.appendChild(wrap);
 }