feat(view): 지형 모델이 없을 때 교각별 지반 패드를 정사각형으로 표시

- 한 변 = 코핑 길이 + 2 m, 교각 축에 맞춰 코핑 중심에 둠, 지반고 높이
- 패드는 교각별 추정이며 서로 잇지 않음 (지형 면 아님)
- 테스트: 상면 네 모서리로 한 변과 대각선 길이 확인

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

tests/ground-marker.test.ts

@@ -10,18 +10,27 @@ const model = buildPier(loadP1());
 const terrain: TerrainGrid = { origin: [259815.676, 179088.094], step: 5, nx: 2, ny: 2, z: [30, 31, 32, 33] };
 
 describe('지반 표시 (지형 모델이 없을 때)', () => {
-  it('draws a thin ground-level line at the pier 지반고, not a 60 m square', () => {
+  it('draws a thin square pad at the pier 지반고, side = coping length + 2 m', () => {
     const scene = buildPierScene(model);
+    const mesh = scene.parts.ground.children[0] as THREE.Mesh;
+    mesh.updateMatrixWorld(true);
+    const pos = mesh.geometry.getAttribute('position');
+    const pts: THREE.Vector3[] = [];
+    for (let i = 0; i < pos.count; i++) pts.push(new THREE.Vector3().fromBufferAttribute(pos, i).applyMatrix4(mesh.matrixWorld));
+    const top = Math.max(...pts.map((p) => p.y));
+    const corners = pts.filter((p) => Math.abs(p.y - top) < 1e-6);
+    // Four corners of the top face, with a distinct set of positions.
+    const unique = corners.filter((p, i) => corners.findIndex((q) => q.distanceTo(p) < 1e-6) === i);
+    expect(unique).toHaveLength(4);
+    const side = model.copingL + 2;
+    const edges = unique.flatMap((a, i) => unique.slice(i + 1).map((b) => a.distanceTo(b))).sort((a, b) => a - b);
+    // Four sides of length `side`, two diagonals of length side·√2.
+    for (const e of edges.slice(0, 4)) expect(e).toBeCloseTo(side, 3);
+    for (const e of edges.slice(4)) expect(e).toBeCloseTo(side * Math.SQRT2, 3);
+    // Thin: a pad, not a solid block. Centred on the ground EL.
     const box = new THREE.Box3().setFromObject(scene.parts.ground);
-    const size = box.getSize(new THREE.Vector3());
-    // Thin: a line, not a plane. Centred on the ground EL.
-    expect(size.y).toBeLessThan(0.1);
+    expect(box.getSize(new THREE.Vector3()).y).toBeLessThan(0.1);
     expect(box.getCenter(new THREE.Vector3()).y).toBeCloseTo(model.groundEl, 2);
-    // Spans the coping along the pier, plus a small margin. The line may be rotated to the 교축, so measure its
-    // horizontal extent as a diagonal of the box's x and z extents.
-    const plan = Math.hypot(size.x, size.z);
-    expect(plan).toBeGreaterThan(model.copingL);
-    expect(plan).toBeLessThan(model.copingL + 3);
   });
 
   it('keeps the ground group out of the pier size', () => {