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>

src/view/scene.ts

@@ -20,8 +20,8 @@ export const PART_LABELS: Record<PartName, string> = {
   ground: '지반',
 };
 
-/** Ground-level line drawn without a terrain grid (m): length = coping + margin, thin and narrow. */
-const GROUND_LINE = { margin: 2, thickness: 0.05, width: 0.15 } as const;
+/** Ground pad drawn without a terrain grid (m): a square of side = coping length + margin, thin. */
+const GROUND_PAD = { margin: 2, thickness: 0.05 } as const;
 
 const COLORS: Record<PartName, number> = {
   coping: 0xb8c2cc,
@@ -177,15 +177,17 @@ export function buildPierScene(m: PierModel, terrain?: TerrainGrid): PierScene {
   }
 
   // Ground: the terrain grid around the piers when the ground model has one (수 m 떨어진 지형까지 보임).
-  // Without a terrain, only a thin line at this pier's 지반고 across the coping (no surface is assumed, 승인 2026-10-27).
+  // Without a terrain, a thin square pad at this pier's 지반고, side = coping length + 2 m (승인 2026-10-27).
+  // It is a local estimate, not a terrain surface: the pads are not joined to each other.
   if (terrain) {
     const material = new THREE.MeshStandardMaterial({ color: COLORS.ground, roughness: 0.9, metalness: 0, transparent: true, opacity: 0.35, side: THREE.DoubleSide });
     material.userData.baseOpacity = 0.35;
     parts.ground.add(new THREE.Mesh(terrainGeometry(terrain, m.pivot), material));
   } else {
-    const line = new THREE.BoxGeometry(m.copingL + GROUND_LINE.margin, GROUND_LINE.thickness, GROUND_LINE.width);
-    line.translate(m.copingL / 2, m.groundEl, 0);
-    add('ground', line, 1);
+    const side = m.copingL + GROUND_PAD.margin;
+    const pad = new THREE.BoxGeometry(side, GROUND_PAD.thickness, side);
+    pad.translate(m.copingL / 2, m.groundEl, 0);
+    add('ground', pad, 0.35);
   }
 
   const root = new THREE.Group();