feat(rebar): 계단부 C15·C16을 모델의 실제 계단으로 만든다 (T형·다주형 공통)

T형은 P1 문서의 고정 위치(2.196~12.000 m)·높이·11개를 써서 코핑 길이가 다르면 철근이 코핑 밖으로 나갔고,
다주형은 높은 계단 전체에 C15 하나만 두었다. 공통 규칙 (step-bars.ts):
- 열마다(전·후열 단차가 있으면 각 쪽) 최저 상면 + h_16(100) 이상 구간이 계단, 경계는 mm 반올림
- 이웃보다 모두 높은 구간 C15(⊓), 나머지 계단 C16(L형, 낮은 쪽 다리, 높은 쪽으로 l_d 연장)
- 폭 방향: T형은 주철근 내부 격자 하나 건너, 다주형은 상면 주철근 하나 건너
- 양쪽이 높은 계단은 RB-S01 알림, RB-M04 폐지. 꺾임은 편 좌표의 한 개(꺾은 철근)
P1 문서값(C15 11 × 3852, C16 22 × 4070/4069, 137.5 · 10@250)을 그대로 재현한다.

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

tests/step-bars.test.ts

@@ -0,0 +1,86 @@
+import { describe, expect, it } from 'vitest';
+import { buildPier } from '../src/core/geometry';
+import { tStepBars } from '../src/core/rebar/coping';
+import { multicolCopingInfo } from '../src/core/rebar/multicol-coping';
+import type { PierInput } from '../src/core/types';
+import { loadP1 } from './helpers';
+
+// 계단부 C15 · C16 일반 규칙 (결정 2026-11-10, src/core/rebar/step-bars.ts). P1 문서값은 rebar-stairs.test.ts.
+
+/** P1 with the 교좌면 EL (and z with it) of bearings 1–5 / 6–10 set to `seats` (m), both rows alike. */
+function withSeats(seats: number[]): PierInput {
+  const p1 = loadP1();
+  return {
+    ...p1,
+    bearings: p1.bearings.map((b) => {
+      const s = seats[(b.no - 1) % 5];
+      return { ...b, z: b.z - b.seatEl! + s, seatEl: s };
+    }),
+  };
+}
+const SEAT = [43.411, 43.483, 43.555, 43.626, 43.698];
+
+describe('계단부 철근 일반 규칙', () => {
+  it('steps that fall towards the end: C15 on the first step (leg at the coping start + c), C16 legs at the low (end) side', () => {
+    const m = buildPier(withSeats([...SEAT].reverse()));
+    const bars = tStepBars(m).bars;
+    const c15 = bars.filter((b) => b.name === 'C15');
+    const c16 = bars.filter((b) => b.name === 'C16');
+    expect(c15.length).toBe(11);
+    expect(c15[0].x0).toBe(100);
+    expect(c16.length).toBe(22);
+    expect(c16.every((b) => b.legs === 'end')).toBe(true);
+    // Mirror of P1: the run reaches l_d into the higher (start-side) step.
+    expect(c16.map((b) => b.length).sort()).toEqual([...Array(11).fill(4069), ...Array(11).fill(4070)].sort());
+  });
+
+  it('every bar stays inside the coping length (the P1 positions no longer leak past a shorter coping)', () => {
+    const m = buildPier({ ...withSeats([...SEAT].reverse()), copingLengthMm: 12000 });
+    const L = m.copingL * 1000;
+    for (const b of tStepBars(m).bars) {
+      expect(b.x0).toBeGreaterThanOrEqual(100);
+      expect(b.x1).toBeLessThanOrEqual(L - 100);
+    }
+  });
+
+  it('no step bar when no step is h_16 (100 mm) above the lowest top', () => {
+    const m = buildPier(withSeats([43.411, 43.45, 43.48, 43.5, 43.51]));
+    expect(tStepBars(m).bars).toEqual([]);
+  });
+
+  it('a step with both neighbours higher gets no bar and a warning (RB-S01 규칙 밖)', () => {
+    const m = buildPier(withSeats([43.698, 43.561, 43.711, 43.411, 43.411]));
+    const { bars, warnings } = tStepBars(m);
+    expect(warnings).toHaveLength(1);
+    expect(warnings[0]).toContain('양쪽이 더 높아');
+    // The two peaks each get C15; the valley step none.
+    expect(bars.filter((b) => b.name === 'C15').length).toBe(22);
+    expect(bars.filter((b) => b.name === 'C16')).toEqual([]);
+  });
+
+  it('with a lateral step, front and back rows are separate and stay in their own part of the width', () => {
+    const p1 = loadP1();
+    const bearings = p1.bearings.map((b) => (b.row === '후열' && b.no === 7 ? { ...b, z: b.z + 0.3, seatEl: b.seatEl! + 0.3 } : b));
+    const m = buildPier({ ...p1, bearings });
+    const [front, back] = m.copingParts.filter((p) => p.k === 0);
+    const bars = tStepBars(m).bars;
+    expect(new Set(bars.map((b) => b.row))).toEqual(new Set(['전열', '후열']));
+    for (const b of bars) {
+      const part = b.row === '전열' ? front : back;
+      expect(b.n).toBeGreaterThanOrEqual(part.nFrom - 1e-9);
+      expect(b.n).toBeLessThanOrEqual(part.nTo + 1e-9);
+    }
+    // The raised back segment 2 (⑦) is a peak: C15 there, on the back row only.
+    const backC15 = bars.filter((b) => b.row === '후열' && b.name === 'C15');
+    expect(new Set(backC15.map((b) => b.x0))).toEqual(new Set([2296, 9812]));
+  });
+
+  it('multi-column copings use the same rule: a peak gets C15, a lower step C16 (no more RB-M04)', () => {
+    const m = buildPier({ ...withSeats(SEAT), type: '2주형', columnCount: 2, foundationType: '말뚝기초', groundEl: 30, pileLengthMm: 24000 });
+    const info = multicolCopingInfo(m);
+    const names = info.bars.map((b) => b.name);
+    expect(names).toContain('C15');
+    expect(names).toContain('C16');
+    expect(info.warnings.some((w) => w.code === 'RB-M04')).toBe(false);
+  });
+});