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/column.ts

@@ -0,0 +1,123 @@
+import { developmentLength, hookLength, NOMINAL_DIAMETER } from './common';
+
+/**
+ * 기둥 철근 C4 (plan C4). Source: RAG/기둥철근 (1).md, 2026-10-22 최종 정리. Type-1 원형 중실 기둥.
+ * Lengths in mm, heights in mm from the 확대기초 상면 (up +).
+ */
+export interface ColumnInput {
+  /** 기둥 지름 D. */
+  D: number;
+  /** 기둥 높이 H_c (확대기초 상면 ~ 코핑 Level 하면). */
+  Hc: number;
+  /** 기둥 중심 위치의 코핑 높이 h_cop. */
+  hcop: number;
+  /** 기둥 면 ~ P1 중심 피복 c. */
+  c: number;
+  /** P1 위 끝의 코핑 상면 아래 깊이 e_P1. */
+  eP1: number;
+  /** 띠철근 구간 위 끝의 코핑 상면 아래 깊이 e_top. */
+  eTop: number;
+  /** 확대기초 깊이 H_f (for P4). */
+  Hf: number;
+  /** 주철근 원주 등분 수 n_div. */
+  nDiv: number;
+  /** 주철근 커플러 간격 (확대기초 상면부터). */
+  ctcCoupler: number;
+  /** P2 커플러를 P1보다 올리는 높이. */
+  dzCp2: number;
+  ctcP3: number;
+  ctcP5: number;
+  ctcP4: number;
+  /** P4 구간 깊이 비율 r_P4. */
+  rP4: number;
+  ctcP6: number;
+}
+
+/** 예제 운곡교 P1 (기둥철근.md §1.2 기본값). */
+export const COLUMN_P1: ColumnInput = {
+  D: 2000,
+  Hc: 12703,
+  hcop: 2644,
+  c: 125,
+  eP1: 200,
+  eTop: 1623,
+  Hf: 2000,
+  nDiv: 46,
+  ctcCoupler: 3000,
+  dzCp2: 150,
+  ctcP3: 300,
+  ctcP5: 120,
+  ctcP4: 120,
+  rP4: 0.5,
+  ctcP6: 1800,
+};
+
+const dS32 = NOMINAL_DIAMETER.S32;
+const dH19 = NOMINAL_DIAMETER.H19;
+const dH16 = NOMINAL_DIAMETER.H16;
+const round = (v: number, d = 1) => Math.round(v * 10 ** d) / 10 ** d;
+
+export function columnRebar(x: ColumnInput = COLUMN_P1) {
+  // P1 and P2 main bars (기둥철근.md §3)
+  const r1 = x.D / 2 - x.c;
+  const r2 = r1 - dS32;
+  const zTop1 = x.Hc + x.hcop - x.eP1;
+  const lD2 = developmentLength(dS32, 500, 27);
+  const zTop2 = x.Hc + lD2;
+  const couplerLevels = Array.from({ length: Math.max(0, Math.floor(zTop1 / x.ctcCoupler) - 1) }, (_, i) => (i + 1) * x.ctcCoupler);
+  const couplersP1 = couplerLevels.length;
+  const mainLines = x.nDiv;
+  const mainP1 = mainLines * (couplersP1 + 1);
+  const mainP2 = mainLines * (couplersP1 + 1);
+  const couplers = couplersP1 * x.nDiv * 2;
+
+  // Ties (기둥철근.md §4–§6)
+  // Tie radius r_t = r_1 + (d_P1 + d_P3)/2 (기둥철근.md §4.1)
+  const tieRadius = r1 + (dS32 + dH19) / 2;
+  const zTopT = x.Hc + x.hcop - x.eTop;
+  const lEmin = Math.max(x.D, zTopT / 6, 450);
+  const n5 = Math.floor(lEmin / x.ctcP5);
+  const lE = Math.max(lEmin, n5 * x.ctcP5 + x.ctcP5 / 2);
+  const p3 = Array.from({ length: Math.floor((zTopT - lE) / x.ctcP3) + 1 }, (_, k) => lE + k * x.ctcP3);
+  const n5all = Math.floor(lE / x.ctcP5);
+  const p5 = Array.from({ length: n5all }, (_, j) => lE - (j + 1) * x.ctcP5);
+  const z5last = lE - n5all * x.ctcP5;
+  const m4 = Math.floor((z5last + x.rP4 * x.Hf) / x.ctcP4);
+  const p4 = Array.from({ length: m4 }, (_, j) => z5last - (j + 1) * x.ctcP4);
+
+  // P6 crossties (기둥철근.md §7): 4 bars per 단, from the first P4
+  const z4first = p4.length ? p4[0] : z5last;
+  const p6Levels: number[] = [];
+  for (let z = z4first; z <= x.Hc; z += x.ctcP6) p6Levels.push(z);
+  const p6Straight = 2 * tieRadius;
+  const p6Hook = hookLength('tie90', dH16);
+  const p6Length = p6Straight + 2 * p6Hook;
+
+  return {
+    r1: round(r1),
+    r2: round(r2),
+    angleStepDeg: 360 / x.nDiv,
+    zTop1: round(zTop1),
+    zTop2: round(zTop2),
+    couplerP1: couplerLevels,
+    couplerP2: couplerLevels.map((z) => z + x.dzCp2),
+    mainP1,
+    mainP2,
+    couplers,
+    tieRadius: round(tieRadius, 2),
+    tieCircumference: round(2 * Math.PI * tieRadius, 1),
+    zTopT: round(zTopT),
+    lEmin: round(lEmin),
+    lE: round(lE),
+    p3,
+    p5,
+    p4,
+    p6Levels,
+    p6Count: p6Levels.length * 4,
+    p6Length: round(p6Length, 1),
+    p4SemicircleLength: round(Math.PI * tieRadius, 1),
+    p3Count: p3.length,
+    p5Count: p5.length,
+    p4Count: p4.length,
+  };
+}