feat(abut): 벽체·흉벽 평면을 skew로 변경

- 벽체 블록, 흉벽 배면 단차부, 흉벽 본체를 날개벽 선 방향(s = s_axis + (ws/wt)·t)으로 밀린 프리즘으로 만듦
- 벽체 전면·배면이 저판과 같은 skew 평행사변형을 이루고, 날개벽 바깥 선과 맞닿음
- 테스트: 같은 교좌면 단계 안에서 벽체와 흉벽이 공통 체적 없이 맞닿음 (인접 단계는 경계 상자만 겹치므로 제외). 전체 641건 통과, 타입 검사·빌드 통과. 브라우저 시각 확인은 미실시

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

src/view/abut-scene.ts

@@ -163,6 +163,19 @@ export function buildAbutGroup(
   };
 
   const footingTop = members.footingTopEl;
+  // Skew (2026-11-03): the 벽체·흉벽 plan follows the wing lines, s = s_axis + k·t, with k = ws/wt of the wing direction.
+  const skewAxis = wingDirectionAxis(model);
+  const skewK = skewAxis.ws / skewAxis.wt;
+  // A prism whose profile (s, EL) is given at the face t0 and which is extruded along t with the skew (λ·(k, 0, 1)).
+  const skewPrism = (profile: [number, number][], depth: number, t0: number) =>
+    prism(
+      profile.map(([s, y]) => [s + skewK * t0, y] as [number, number]),
+      depth,
+      new THREE.Vector3(0, 0, t0),
+      ex,
+      ey,
+      new THREE.Vector3(skewK, 0, 1),
+    );
   const footingBottom = members.footingBottomEl;
 
   // 벽체 (spec §4.3, 삽도): the full 2400 block from the footing top up to the 교좌면 step (t −0.8 … 1.6).
@@ -175,7 +188,7 @@ export function buildAbutGroup(
       [seg.sTo, seg.seatEl],
       [seg.sFrom, seg.seatEl],
     ];
-    add('wall', prism(block, members.shape.wallFrontM - members.shape.wallBackM, new THREE.Vector3(0, 0, members.shape.wallBackM), ex, ey, ez), 0xb7c7d3);
+    add('wall', skewPrism(block, members.shape.wallFrontM - members.shape.wallBackM, members.shape.wallBackM), 0xb7c7d3);
   }
 
   // 흉벽 (삽도): sits on the 벽체 top, above the 교좌면 step. The back strip (t −0.8 … −0.5) rises to the 브라켓 상면 (흉벽 상단 − 0.85),
@@ -187,21 +200,19 @@ export function buildAbutGroup(
       [seg.sTo, bracketAt(seg.sTo)],
       [seg.sFrom, bracketAt(seg.sFrom)],
     ];
-    add('parapet', prism(backStrip, members.shape.parapetBackM - members.shape.wallBackM, new THREE.Vector3(0, 0, members.shape.wallBackM), ex, ey, ez), 0x8898aa);
+    add('parapet', skewPrism(backStrip, members.shape.parapetBackM - members.shape.wallBackM, members.shape.wallBackM), 0x8898aa);
     const parapet: [number, number][] = [
       [seg.sFrom, seg.seatEl],
       [seg.sTo, seg.seatEl],
       [seg.sTo, parapetTopAt(seg.sTo)],
       [seg.sFrom, parapetTopAt(seg.sFrom)],
     ];
-    add('parapet', prism(parapet, -members.shape.parapetBackM, new THREE.Vector3(0, 0, members.shape.parapetBackM), ex, ey, ez), 0x8898aa);
+    add('parapet', skewPrism(parapet, -members.shape.parapetBackM, members.shape.parapetBackM), 0x8898aa);
   }
 
   if (footingBottom !== undefined) {
     // 저판: s 0 … L, t −1.8 … +2.8, from the 기초 하단 to the 저판 상면.
     // Skew (2026-11-03): the 저판 ends follow the wing outer lines (s = sEnd + (ws/wt)·t), so the plan is a parallelogram.
-    const skewAxis = wingDirectionAxis(model);
-    const skewK = skewAxis.ws / skewAxis.wt;
     const footProfile: [number, number][] = [
       [skewK * members.shape.footingBackM, members.shape.footingBackM],
       [L + skewK * members.shape.footingBackM, members.shape.footingBackM],

tests/abut-blocks-slab.test.ts

@@ -61,9 +61,13 @@ describe('교대 3D: 받침블럭과 접속슬래브 (spec §4.2, §4.8)', () =>
       const box = new THREE.Box3().setFromObject(a).intersect(new THREE.Box3().setFromObject(b));
       return Math.max(0, box.max.x - box.min.x) * Math.max(0, box.max.y - box.min.y) * Math.max(0, box.max.z - box.min.z);
     };
-    for (const w of (named('wall') as THREE.Group).children) {
-      for (const p of (named('parapet') as THREE.Group).children) expect(volume(w, p)).toBeLessThan(1e-9);
-    }
+    // Same 교좌면 step only: the skewed prisms of neighbouring steps share a face, and their bounding boxes overlap slightly.
+    const walls = (named('wall') as THREE.Group).children;
+    const parapets = (named('parapet') as THREE.Group).children;
+    walls.forEach((w, k) => {
+      expect(volume(w, parapets[2 * k])).toBeLessThan(1e-9);
+      expect(volume(w, parapets[2 * k + 1])).toBeLessThan(1e-9);
+    });
     // back face: the ledge (t −0.8 … −0.5) and the 흉벽 back (−0.5) meet at one horizontal edge, not a vertical offset
     const backs = (named('wall') as THREE.Group).children.map((w) => new THREE.Box3().setFromObject(w).min.z);
     expect(Math.min(...backs)).toBeCloseTo(members.shape.wallBackM, 6);