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);
 }

src/view/block-rebar-svg.ts

@@ -1,24 +1,29 @@
 import type { PierModel } from '../core/geometry';
 import { barDepths, barsAcross, type BlockRebarSettings } from '../core/rebar/bearing-block';
 import { INTEGRATED_BLOCK } from '../core/integrated-block';
+import { pairEccentricities } from '../core/bearing-layout';
 
 /**
  * 받침블럭 배근 · 단면 C–C (승인 2026-10-26, 시안 ③): a section along the 교축, through the blocks. x = 교축 (n), y = EL
  * (down from the highest block top). S2 bars are dots along the top, S1 bars are lines with legs down into the coping.
  * Dimensions: the block widths and gap, the bar layout (end · n@spacing = span), and the top cover.
  */
-export function blockRebarSectionSvg(m: PierModel): string {
+export function blockRebarSectionSvg(m: PierModel, pairIndex = 0): string {
   const s: BlockRebarSettings = m.blockRebar;
   const depth = barDepths(s);
-  const ib = m.integratedBlocks;
+  const pair = pairEccentricities(m.blocks)[pairIndex];
+  if (!pair) return '';
+  const f = m.blocks.find((b) => b.no === pair.frontNo && b.row === '전열')!;
+  const k = m.blocks.find((b) => b.no === pair.backNo && b.row === '후열')!;
+  const ib = m.integratedBlocks.find((x) => x.frontNo === pair.frontNo);
   const parts: string[] = [];
-  const topRef = Math.max(...m.blocks.map((b) => b.top));
+  const topRef = Math.max(f.top, k.top, ib?.topFront ?? 0, ib?.topBack ?? 0);
   const Y = (el: number) => (topRef - el) * 1000;
 
-  // Spans to draw: one 2500 span per 일체 pair, else one 1200 span per bearing.
-  const spans = ib.length > 0
-    ? ib.map((x) => ({ nc: x.n, width: INTEGRATED_BLOCK.length, bottom: x.bottom, topL: x.topFront, topR: x.topBack, depthTopL: x.topFront, depthTopR: x.topBack, stepped: true }))
-    : m.blocks.map((b) => ({ nc: b.n, width: 1200, bottom: b.bottom, topL: b.top, topR: b.top, depthTopL: b.top, depthTopR: b.top, stepped: false }));
+  // One section through the selected pair: a 2500 span (일체) or the two 1200 blocks (분리), along the 교축.
+  const spans = ib
+    ? [{ nc: ib.n, width: INTEGRATED_BLOCK.length, bottom: ib.bottom, topL: ib.topFront, topR: ib.topBack, depthTopL: ib.topFront, depthTopR: ib.topBack, stepped: true }]
+    : [f, k].map((b) => ({ nc: b.n, width: 1200, bottom: b.bottom, topL: b.top, topR: b.top, depthTopL: b.top, depthTopR: b.top, stepped: false }));
   const nAll = spans.map((x) => x.nc);
   const nc = (Math.min(...nAll) + Math.max(...nAll)) / 2;
   const X = (n: number) => (n - nc) * 1000;