fix(shape): 꺾임점 근처 계단 경계를 꺾임점에 맞춘다

계단 경계가 꺾임점에서 블럭 반폭(0.6 m) 이내이면 경계를 꺾임점으로 옮겨, 계단면이 미터면과 같아져 두 구간이
쐐기 틈 없이 만난다 (산외교 P11 후열 ⑥|⑦: 15.07 → 15.14). 양쪽 받침이 꺾임점에서 블럭 반폭 이상 떨어질 때만 옮긴다.

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

tests/kink-steps.test.ts

@@ -0,0 +1,55 @@
+import { describe, expect, it } from 'vitest';
+import { readFileSync } from 'node:fs';
+import { fileURLToPath } from 'node:url';
+import * as XLSX from 'xlsx';
+import { parseWorkbook } from '../src/core/parseWorkbook';
+import { buildPier, DIM, defaultLevelWidthMm } from '../src/core/geometry';
+import type { PierInput } from '../src/core/types';
+
+// 산외교 P11 (샘플01) with the coping bent 8.767° at IP4 (2026-11-10 검토): back ⑥ g 11.64, ⑦ g 18.49, kink g 15.14.
+const SAMPLE = fileURLToPath(new URL('../project_data/01_엑셀/샘플01_산외교_교각입력데이터.xlsx', import.meta.url));
+
+function p11(): PierInput {
+  const wb = XLSX.read(readFileSync(SAMPLE), { type: 'buffer' });
+  wb.Sheets['IP']['F46'] = { t: 'n', v: 8.767 };
+  const input = parseWorkbook(wb).piers.find((p) => p.name === 'P11')!;
+  return { ...input, levelWidthMm: input.levelWidthMm ?? defaultLevelWidthMm(input.type, 2000) };
+}
+
+const kinkOf = (m: ReturnType<typeof buildPier>) => m.stretches[1].base;
+const gOf = (m: ReturnType<typeof buildPier>, no: number) => m.bearings.find((b) => b.no === no)!.g;
+
+describe('꺾임점 근처 계단 경계 (A, 2026-11-10)', () => {
+  it('a back boundary 0.07 m from the kink moves onto the kink (⑥|⑦)', () => {
+    const m = buildPier(p11());
+    const kink = kinkOf(m);
+    const own = (gOf(m, 6) + gOf(m, 7)) / 2;
+    expect(Math.abs(own - kink)).toBeLessThan(0.1);
+    expect(m.backSegments![2].s0).toBeCloseTo(kink, 9);
+    expect(m.backSegments![1].s1).toBeCloseTo(kink, 9);
+  });
+
+  it('boundaries far from the kink stay midway between their bearings', () => {
+    const m = buildPier(p11());
+    expect(m.backSegments![1].s0).toBeCloseTo((gOf(m, 5) + gOf(m, 6)) / 2, 9);
+    expect(m.backSegments![3].s0).toBeCloseTo((gOf(m, 7) + gOf(m, 8)) / 2, 9);
+    m.segments.slice(1).forEach((seg) => expect(Math.abs(seg.s0 - kinkOf(m))).toBeGreaterThan(DIM.blockSize / 2));
+  });
+
+  it('no snap when a bearing would end up closer than half a block to the kink', () => {
+    const input = p11();
+    const base = buildPier(input);
+    // ⑦ 0.4 m past the kink, ⑥ 0.6 m before it: midpoint 0.1 m before the kink, but ⑦ is within half a block.
+    const u1 = base.stretches[1].u;
+    const u0 = base.stretches[0].u;
+    const kink = kinkOf(base);
+    const d7 = kink + 0.4 - gOf(base, 7);
+    const d6 = kink - 0.6 - gOf(base, 6);
+    const bearings = input.bearings.map((b) =>
+      b.no === 7 ? { ...b, x: b.x + d7 * u1.e, y: b.y + d7 * u1.n } : b.no === 6 ? { ...b, x: b.x + d6 * u0.e, y: b.y + d6 * u0.n } : b);
+    const m = buildPier({ ...input, bearings });
+    const own = (gOf(m, 6) + gOf(m, 7)) / 2;
+    expect(kinkOf(m) - own).toBeCloseTo(0.1, 3);
+    expect(m.backSegments![2].s0).toBeCloseTo(own, 9);
+  });
+});