fix(shape): 후열 계단 경계를 후열 받침 사이 중앙에 둔다

전·후열 받침 수가 같으면 후열 계단 경계를 전열 경계로 썼기 때문에, 후열 받침이 전열과 마주 보지 않는 교각
(산외교 P11 ⑥|⑦: 경계 12.75, 받침 11.64 · 18.49)에서 경계가 중앙에서 벗어났다. 후열도 자기 받침 사이 중앙을 쓰되,
전열 경계와 블럭 반폭(0.6 m) 이내로 가까우면 전열 경계에 맞춘다 (P1처럼 약간 엇갈린 열에 얇은 단이 생기지 않게).

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

src/core/geometry.ts

@@ -344,9 +344,16 @@ export function buildPier(input: PierInput, ctx: PierContext = {}): PierModel {
   const segments: Segment[] = frontPlaced.map((b, k) => ({ s0: bounds[k], s1: bounds[k + 1], top: seatOf.get(b.no) ?? b.z - h1 }));
 
   // Lateral step (spec §4.3, Q1·Q2 추천안): front and back tops differ by 5 mm or more in some segment.
+  // The back steps break midway between consecutive back bearings (2026-11-10), as the front ones do: back bearings need
+  // not sit opposite the front ones (산외교 P11 ⑥ ↔ ⑦). With equal counts, a back bound less than half a block from the
+  // front bound snaps to it, so slightly staggered rows (P1, ≈ 0.09 m) keep one boundary and no sliver step.
+  const backBound = (k: number) => {
+    const own = (backPlaced[k-1].g + backPlaced[k].g) / 2;
+    return !unequalRows && Math.abs(own - bounds[k]) < DIM.blockSize / 2 ? bounds[k] : own;
+  };
   const backSegments: Segment[] = backPlaced.map((b,k) => ({
-    s0: unequalRows ? (k === 0 ? 0 : (backPlaced[k-1].g+b.g)/2) : segments[k].s0,
-    s1: unequalRows ? (k === backPlaced.length-1 ? L : (b.g+backPlaced[k+1].g)/2) : segments[k].s1,
+    s0: k === 0 ? 0 : backBound(k),
+    s1: k === backPlaced.length-1 ? L : backBound(k + 1),
     top: seatOf.get(b.no) ?? b.z-h2,
   }));
   const stepBounds = [...new Set([0,L,...segments.slice(1).map(s=>s.s0),...backSegments.slice(1).map(s=>s.s0)])].sort((a,b)=>a-b);

tests/validate-and-foundation.test.ts

@@ -255,6 +255,27 @@ describe('횡방향 단차 (spec §4.3, Q1·Q2 추천안)', () => {
     }
   });
 
+  it('slightly staggered rows (P1, < half a block) keep the front bounds for the back steps', () => {
+    const m = buildPier(loadP1());
+    expect(m.backSegments!.map((x) => x.s0)).toEqual(m.segments.map((x) => x.s0));
+  });
+
+  it('back steps break midway between back bearings, even when they are not opposite the front ones (2026-11-10)', () => {
+    const p1 = loadP1();
+    // Move back bearing 7 by 1.5 m along the row: its bounds follow it, the front bounds do not.
+    const m0 = buildPier(p1);
+    const u = m0.u;
+    const bearings = p1.bearings.map((b) => (b.no === 7 ? { ...b, x: b.x + 1.5 * u.e, y: b.y + 1.5 * u.n } : b));
+    const m = buildPier({ ...p1, bearings });
+    const back = m.bearings.filter((b) => b.row === '후열').sort((a, b) => a.g - b.g);
+    const bs = m.backSegments!.map((x) => x.s0);
+    // ⑥|⑦ and ⑦|⑧ move 0.75 m (> half a block): own midpoints. ⑧|⑨ and ⑨|⑩ stay on the front bounds.
+    expect(bs[1]).toBeCloseTo((back[0].g + back[1].g) / 2, 9);
+    expect(bs[2]).toBeCloseTo((back[1].g + back[2].g) / 2, 9);
+    expect(bs.slice(3)).toEqual(m.segments.slice(3).map((x) => x.s0));
+    expect(m.segments.map((x) => x.s0)).toEqual(m0.segments.map((x) => x.s0));
+  });
+
   it('a 3 mm difference is below the 5 mm tolerance: no step', () => {
     const p1 = loadP1();
     const bearings = p1.bearings.map((b) => (b.row === '후열' && b.no === 7 ? { ...b, z: b.z + 0.003 } : b));