fix(rebar): 상면 단 가로 간격을 d(H29, 28.6 mm)로 수정

- 단마다 양끝이 안으로 들어가는 양을 2d에서 d로 변경
- 상면 단 길이 12744 / 12686.8 / 12629.6 (기존 12744 / 12629.6 / 12515.2)
- 3D 상면 주철근의 굽힘 시작점도 같은 값을 쓴다

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

src/view/rebar3d.ts

@@ -146,7 +146,7 @@ export function buildRebar3D(m: PierModel, common: Material = DEFAULT_MATERIAL,
     const cover = 0.1;
     // Top rows C1–C6 (코핑철근.md §5.1, §5.3): counted from the left, the even (1-based) positions are cycle 1 (C1–C3),
     // the odd cycle 2 (C4–C6). In 0-based j that is odd j → cycle 1. Each row turns down 472 at both ends, and the
-    // k-th row's ends sit (k − 1)·57.2 further in.
+    // k-th row's ends sit (k − 1)·d (H29, 28.6) further in.
     const topEl = m.copingRefTop;
     const hookTop = MAIN.hook / 1000;
     for (let k = 1; k <= 3; k++) {