fix(shape): 꺾임점 계단면을 미터면과 일치시킨다

계단 경계를 꺾임점에 맞춰도 각 구간의 계단면은 자기 축에 직각이고 미터면은 θ/2 기울어, 코핑 폭 안에서
±(B/2)·tan(θ/2) (P11 ±0.12 m)의 삼각형 조각이 남았다. 꺾임점의 계단면을 각 구간 프로파일에서 미터 퍼짐 + 10 mm만큼
잘려 나가는 쪽으로 옮겨, 구간 0은 앞 상면을, 구간 1은 뒤 상면을 미터면까지 가져가게 한다. 3D와 도면 투영이
모두 미터면으로 자르므로 둘 다 반영된다. 퍼짐이 블럭 반폭 이상이면 바꾸지 않는다.

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

src/core/geometry.ts

@@ -405,6 +405,13 @@ export function buildPier(input: PierInput, ctx: PierContext = {}): PierModel {
   };
   // Each stretch uses the same outline shifted to its own base; the mitre plane trims the overlap at the kink.
   const shifted = (pts: [number, number][], b: number): [number, number][] => pts.map(([s, z]) => [s - b, z]);
+  // A step at the kink (2026-11-10): each stretch's step face is square to its own axis, the mitre is not, so the face
+  // moves past the mitre spread (B/2)·tan(θ/2) into the trimmed side: stretch 0 keeps its left top up to the mitre and
+  // stretch 1 its right top back to it, and the visible step is the mitre plane itself (no sliver on either half).
+  const mitreSpread = (B / 2) * Math.tan((Math.abs(ip4.angleDeg) * Math.PI) / 360) + 0.01;
+  const atKink = (pts: [number, number][], k: number): [number, number][] =>
+    kinkG === null || mitreSpread >= halfBlock ? pts
+      : pts.map(([s, z]) => [Math.abs(s - kinkG) < 1e-9 ? kinkG + (k === 0 ? mitreSpread : -mitreSpread) : s, z]);
 
   const meanN = (rows: { n: number }[]) => rows.reduce((sum, r) => sum + r.n, 0) / rows.length;
   const copingParts: CopingPart[] = [];
@@ -424,11 +431,11 @@ export function buildPier(input: PierInput, ctx: PierContext = {}): PierModel {
       // side of its own row; the order stays [front, back].
       const frontLow = (fRows.length && bRows.length ? meanN(fRows) - meanN(bRows) : meanN(frontPlaced) - meanN(backPlaced)) <= 0;
       copingParts.push(
-        { k, nFrom: frontLow ? -planHalfB : nStep, nTo: frontLow ? nStep : planHalfB, profile: shifted(outline(segments), base[k]) },
-        { k, nFrom: frontLow ? nStep : -planHalfB, nTo: frontLow ? planHalfB : nStep, profile: shifted(outline(backSegments), base[k]) },
+        { k, nFrom: frontLow ? -planHalfB : nStep, nTo: frontLow ? nStep : planHalfB, profile: shifted(atKink(outline(segments), k), base[k]) },
+        { k, nFrom: frontLow ? nStep : -planHalfB, nTo: frontLow ? planHalfB : nStep, profile: shifted(atKink(outline(backSegments), k), base[k]) },
       );
     } else {
-      copingParts.push({ k, nFrom: -planHalfB, nTo: planHalfB, profile: shifted(outline(segments), base[k]) });
+      copingParts.push({ k, nFrom: -planHalfB, nTo: planHalfB, profile: shifted(atKink(outline(segments), k), base[k]) });
     }
   }
 

tests/kink-steps.test.ts

@@ -89,3 +89,29 @@ describe('전·후열 나눔선 하나 (B, 2026-11-10)', () => {
     for (const k of [0, 1]) expect(split(m, k)).toBeCloseTo((meanN(m, '전열', k) + meanN(m, '후열', k)) / 2, 9);
   });
 });
+
+describe('꺾임점 계단면 = 미터면 (2026-11-10)', () => {
+  // Top of a stepped profile at s: the last top point at or before s.
+  const topAt = (m: ReturnType<typeof buildPier>, profile: [number, number][], s: number) =>
+    profile.slice(0, profile.findIndex(([ps]) => ps > s + 1e-9)).filter(([, z]) => z > m.endBottom + 1e-9).pop()![1];
+
+  it('over the whole mitre spread, stretch 0 keeps the top before the kink and stretch 1 the top after it', () => {
+    const m = buildPier(p11());
+    const kink = kinkOf(m);
+    const spread = (m.copingB / 2) * Math.tan((8.767 * Math.PI) / 360);
+    // The back part carries the ⑥|⑦ step at the kink: ⑥ top (back segment 1) before, ⑦ top (segment 2) after.
+    expect(m.backSegments![2].s0).toBeCloseTo(kink, 9);
+    const before = m.backSegments![1].top, after = m.backSegments![2].top;
+    expect(after - before).toBeGreaterThan(0.5);
+    const backPart = (k: number) => {
+      const parts = m.copingParts.filter((p) => p.k === k);
+      return parts.find((p) => p.profile.some(([, z]) => Math.abs(z - after) < 1e-9))!;
+    };
+    // Stretch 0 (local s = g): just short of the far edge of the mitre spread, still the ⑥ top.
+    expect(topAt(m, backPart(0).profile, kink + spread - 1e-6)).toBeCloseTo(before, 9);
+    // Stretch 1 (local s = g − kink): just past the near edge of the spread, already the ⑦ top.
+    expect(topAt(m, backPart(1).profile, -spread + 1e-6)).toBeCloseTo(after, 9);
+    // Away from the kink the profiles are unchanged: the ⑦ top in stretch 0 a block past the kink.
+    expect(topAt(m, backPart(0).profile, kink + DIM.blockSize / 2 + 0.1)).toBeCloseTo(after, 9);
+  });
+});