feat(abut): 저판 평면을 skew 평행사변형으로 변경

- 저판 양 끝을 날개벽 바깥 선(s = sEnd + (ws/wt)·t)을 따라 잘라, 평면이 직사각형이 아니라 skew 평행사변형이 되도록 함
- 날개벽 하면이 저판 가장자리와 맞닿음
- 테스트: 저판 뒷면 모서리가 날개벽 바깥 선 위에 있음. 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

@@ -199,11 +199,14 @@ export function buildAbutGroup(
 
   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][] = [
-      [0, members.shape.footingBackM],
-      [L, members.shape.footingBackM],
-      [L, members.shape.footingFrontM],
-      [0, members.shape.footingFrontM],
+      [skewK * members.shape.footingBackM, members.shape.footingBackM],
+      [L + skewK * members.shape.footingBackM, members.shape.footingBackM],
+      [L + skewK * members.shape.footingFrontM, members.shape.footingFrontM],
+      [skewK * members.shape.footingFrontM, members.shape.footingFrontM],
     ];
     add('footing', prism(footProfile, members.shape.footingThicknessM, new THREE.Vector3(0, footingBottom, 0), ex, ez, ey), 0x5a6a78);
   }

tests/abut-blocks-slab.test.ts

@@ -126,4 +126,23 @@ describe('교대 3D: 받침블럭과 접속슬래브 (spec §4.2, §4.8)', () =>
       }
     }
   });
+
+  it('the 저판 plan is a skew parallelogram: its back corners sit on the wing outer lines, so the 날개벽 sits flush on the 저판 edge', () => {
+    const axis = (p: { e: number; n: number }) => {
+      const d = { e: p.e - model.ip1.e, n: p.n - model.ip1.n };
+      const f = { e: model.nLeft.e * model.frontSide, n: model.nLeft.n * model.frontSide };
+      return { s: d.e * model.u.e + d.n * model.u.n, t: d.e * f.e + d.n * f.n };
+    };
+    const p0 = axis(wings[0].corners[0]);
+    const p1 = axis(wings[0].corners[1]);
+    const k = (p1.s - p0.s) / (p1.t - p0.t); // s per m of t along the wing
+    const pos = ((named('footing') as THREE.Group).children[0] as THREE.Mesh).geometry.getAttribute('position');
+    const back = [...Array(pos.count).keys()].map((i) => ({ x: pos.getX(i), z: pos.getZ(i) })).filter((p) => Math.abs(p.z - members.shape.footingBackM) < 1e-6);
+    expect(back.length).toBeGreaterThanOrEqual(2);
+    for (const p of back) {
+      const onLeft = Math.abs(p.x - k * p.z) < 1e-6;
+      const onRight = Math.abs(p.x - (model.length + k * p.z)) < 1e-6;
+      expect(onLeft || onRight).toBe(true);
+    }
+  });
 });