feat(core): coping main bars C1–C8 (plan C3) and column rebar C4

- Nominal bar diameters (H29 28.6, S32 31.8, …) as used by the documents. With
  them, C7/C8 l_s = 1.3·l_d(28.6, fck 27) = 1717.6, which is the document's 1717.
- C1–C8 (코핑철근.md §5): width layout 127 · 110.5 · 21@125 · 110.5 · 127 with 24
  bars (12 + 12); heights 43.284 / 43.227 / 43.170; top bar lengths 12744 /
  12629.6 / 12515.2; bottom pieces ≈ 7448.
- C4 (기둥철근.md): P1 r 875 and P2 r 843.2; couplers P1 +3/6/9/12 m, P2 +150;
  230 + 230 main pieces and 368 couplers; tie radius 900.45; P3 38 at @300 from
  2.340; P5 19 at @120; P4 8 at @120 from −0.060; P6 8 levels × 4 = 32, length
  ≈ 2071.
- Not yet: C4 interference checks (P4 vs B11, Q-C9), bar lengths of P3/P5/P4
  and coupler shapes, the coping/column drawing checks, and the ③ screen.

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

src/core/rebar/coping-main.ts

@@ -0,0 +1,80 @@
+import { hookLength, NOMINAL_DIAMETER, spliceLength } from './common';
+
+/**
+ * C1–C8 상·하면 주철근 (plan C3 continued). Source: RAG/코핑철근.md §5, 2026-10-22 최종 정리.
+ * Lengths in mm, heights in m (EL).
+ */
+export const MAIN = {
+  /** 피복 (최외측 철근 중심까지). */
+  cover: 100,
+  /** 기본 간격 s_m and minimum end gap g_min. */
+  spacing: 125,
+  endGapMin: 80,
+  /** 상면 단 간격 s_lay (EL, m) and its horizontal step (mm). */
+  layEl: 0.057,
+  layHorizontal: 57.2,
+  /** 90° 갈고리 l_h (mm, as in the document). */
+  hook: 472,
+  /** 하면 경사 구간 수평 길이 (mm, each side) and Level width. */
+  slopeEach: 5067.8,
+  level: 2100,
+} as const;
+
+export interface WidthLayout {
+  /** Edge bar centre from the coping side face: c + (d_st + d_m)/2. */
+  yE: number;
+  /** Clear length between the two edge bars: B − 2·y_e. */
+  S: number;
+  /** Bars inside the edge bars: n (so n + 3 in all, including the two edges and the two 110.5 ends). */
+  n: number;
+  /** Equal end gap g = (S − n·s_m)/2. */
+  g: number;
+  /** Bars in all: n + 3. */
+  count: number;
+  /** Bars per cycle (alternating): 12 / 12 for 24. */
+  cyclePerKind: number;
+  /** Dimension string, e.g. "127 · 110.5 · 21@125 · 110.5 · 127". */
+  text: string;
+}
+
+/** 폭 방향 배치 (코핑철근.md §5.1). d_st = H25 stirrup, d_m = H29 main. */
+export function widthLayout(B: number, dSt = NOMINAL_DIAMETER.H25, dM = NOMINAL_DIAMETER.H29): WidthLayout {
+  const yE = MAIN.cover + (dSt + dM) / 2;
+  const S = B - 2 * yE;
+  const n = Math.floor((S - 2 * MAIN.endGapMin) / MAIN.spacing);
+  const g = (S - n * MAIN.spacing) / 2;
+  const count = n + 3;
+  const r = (v: number) => Math.round(v * 10) / 10;
+  return {
+    yE: r(yE),
+    S: r(S),
+    n,
+    g: r(g),
+    count,
+    cyclePerKind: count / 2,
+    text: `${r(yE)} · ${r(g)} · ${n}@${MAIN.spacing} · ${r(g)} · ${r(yE)}`,
+  };
+}
+
+/** Top-layer EL of the k-th row (k = 1, 2, 3): z_k = z_1 − (k − 1)·s_lay (m). */
+export function topRowEl(z1: number, k: number): number {
+  return Math.round((z1 - (k - 1) * MAIN.layEl) * 1000) / 1000;
+}
+
+/** Length of one top bar on row k: path L − 2·(c + (k−1)·step), plus two 90° hooks. */
+export function topBarLength(L: number, k: number): number {
+  const path = L - 2 * (MAIN.cover + (k - 1) * MAIN.layHorizontal);
+  return path + 2 * MAIN.hook;
+}
+
+/**
+ * Bottom bar C7·C8 (spliced at the column centre, 하면 겹침이음). Each piece:
+ * path/2 + l_s/2 + l_h, with path = 2·slope + Level and l_s = 1.3·l_d for H29 (nominal 28.6, fck 27, fy 400).
+ */
+export function bottomPiece(fck = 27): { path: number; ls: number; piece: number } {
+  const path = 2 * MAIN.slopeEach + MAIN.level;
+  const ls = spliceLength(NOMINAL_DIAMETER.H29, 400, fck);
+  const lh = hookLength('main90', NOMINAL_DIAMETER.H29);
+  return { path, ls, piece: path / 2 + ls / 2 + lh };
+}
+