fix(shape): 횡방향 단차 코핑을 실제 전열·후열 쪽에 나눈다

코핑 두 쪽(전열 상면 · 후열 상면)을 늘 축의 (−) 쪽 = 전열로 두어, 전열이 (+) 쪽에 있는 교각(P1, 산외교 P11)에서는
전열 블럭이 후열 상면에 묻히고 후열 블럭이 떠 있었다. 각 쪽을 그 열 받침의 평균 n 쪽에 둔다 (순서는 [전열, 후열] 유지).
기존 테스트가 틀린 쪽을 기대하던 것을 고치고, 전열이 어느 쪽이든 블럭이 아래 코핑 상면에 놓이는지 검사한다.

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

tests/validate-and-foundation.test.ts

@@ -219,10 +219,12 @@ describe('횡방향 단차 (spec §4.3, Q1·Q2 추천안)', () => {
     const m = buildPier({ ...p1, bearings });
     expect(m.copingParts).toHaveLength(2);
     const [front, back] = m.copingParts;
-    expect(front.nTo).toBeCloseTo(back.nFrom, 9);
-    expect(front.nFrom).toBeCloseTo(-m.copingB / 2, 9);
-    expect(back.nTo).toBeCloseTo(m.copingB / 2, 9);
-    expect(front.nTo).toBeCloseTo((meanN(m, '전열') + meanN(m, '후열')) / 2, 9);
+    // P1's front row is on the + side of the axis (n ≈ +0.70, back ≈ −0.60), so the front part is the + half (2026-11-10 fix).
+    expect(meanN(m, '전열')).toBeGreaterThan(meanN(m, '후열'));
+    expect(front.nFrom).toBeCloseTo(back.nTo, 9);
+    expect(front.nTo).toBeCloseTo(m.copingB / 2, 9);
+    expect(back.nFrom).toBeCloseTo(-m.copingB / 2, 9);
+    expect(front.nFrom).toBeCloseTo((meanN(m, '전열') + meanN(m, '후열')) / 2, 9);
     // Segment 2 (bearing no. 7) is the one raised by 20 mm on the back row only.
     const topAt = (part: typeof front, s: number) => part.profile.find(([ps, pz]) => Math.abs(ps - s) < 1e-9 && pz > m.levelBottom)![1];
     const s2 = m.segments[2].s0;
@@ -235,6 +237,24 @@ describe('횡방향 단차 (spec §4.3, Q1·Q2 추천안)', () => {
     expect(buildPier({ ...p1, bearings }).copingParts).toHaveLength(2);
   });
 
+  it('each block sits on the coping part under it, whichever side of the axis the front row is on (2026-11-10)', () => {
+    const p1 = loadP1();
+    const raise = (b: (typeof p1.bearings)[number]) => (b.row === '후열' && b.no === 7 ? { ...b, z: b.z + 0.3, seatEl: b.seatEl! + 0.3 } : b);
+    const swapRows = (b: (typeof p1.bearings)[number]) => ({ ...b, row: (b.row === '전열' ? '후열' : '전열') as typeof b.row });
+    // P1 as given (front on +n) and with the row labels swapped (front on −n).
+    for (const bearings of [p1.bearings.map(raise), p1.bearings.map(raise).map(swapRows)]) {
+      const m = buildPier({ ...p1, bearings });
+      expect(m.copingParts).toHaveLength(2);
+      for (const b of m.blocks) {
+        const part = m.copingParts.find((q) => q.k === b.k && b.n >= q.nFrom && b.n <= q.nTo)!;
+        // The last top point at or before s on the stepped outline is the step top under the block.
+        const before = part.profile.slice(0, part.profile.findIndex(([ps]) => ps > b.s + 1e-9));
+        const top = before.filter(([, pz]) => pz > m.endBottom + 1e-9).pop()![1];
+        expect(b.bottom).toBeCloseTo(top, 6);
+      }
+    }
+  });
+
   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));