fix(bearing): show only the outer block margins as C4 and C5

- A 일체 pair's inner sides face the other bearing, so they are not C4 or C5:
  they are null and shown as "—" in the 배치 규칙 table.
- The table headings now describe the 교축 margins (bearing to the outer block edge).

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

src/main.ts

@@ -1576,13 +1576,14 @@ function renderBearingLayout(wrap: HTMLDivElement, pier: Pier): void {
   const table = el('table', undefined, 'grid-table rebar-table');
   const head = el('thead');
   const hr = el('tr');
-  for (const h of ['받침', '열', 'C4 (코핑 시작 ↔ 받침, mm)', 'C5 (받침 ↔ 코핑 끝, mm)']) hr.appendChild(el('th', h));
+  for (const h of ['받침', '열', 'C4 (교축 · 받침 ↔ 블록 바깥 연단, mm)', 'C5 (교축 · 받침 ↔ 블록 바깥 연단, mm)']) hr.appendChild(el('th', h));
   head.appendChild(hr);
   table.appendChild(head);
   const body = el('tbody');
   for (const e of edges) {
     const tr = el('tr');
-    for (const v of [String(e.no), e.row, e.leftMm.toFixed(0), e.rightMm.toFixed(0)]) tr.appendChild(el('td', v));
+    const fmt = (v: number | null) => (v === null ? '—' : v.toFixed(0));
+    for (const v of [String(e.no), e.row, fmt(e.leftMm), fmt(e.rightMm)]) tr.appendChild(el('td', v));
     body.appendChild(tr);
   }
   table.appendChild(body);