fix(view): 지형 모델이 없을 때 60 m 사각판 대신 교각별 지반고 선 표시

- 지형 격자가 없으면 교각 지반고 위치에 얇은 선(코핑 길이 + 여유 2 m)만 그림
- 전체 보기에서도 지형이 없으면 교각별 지반고 선을 표시, 지형이 있으면 공유 격자만 표시
- 부재 표시 '지반' 체크박스는 두 경우 모두 동작
- 테스트: 얇은 선 위치와 길이, 지형 있을 때 선 미표시

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

src/view/all-piers.ts

@@ -25,7 +25,7 @@ export interface AllPiers {
 
 /**
  * Builds the concrete members of several piers in one frame (전체 보기, 콘크리트만: no rebar). The first entry is the
- * reference. Each pier's own ground plane is hidden; the shared terrain grid is drawn once.
+ * reference. With a terrain grid it is drawn once and the per-pier ground lines are hidden.
  */
 export function buildAllPiers(entries: { name: string; model: PierModel }[], terrain?: TerrainGrid): AllPiers {
   const piers = new THREE.Group();
@@ -33,7 +33,8 @@ export function buildAllPiers(entries: { name: string; model: PierModel }[], ter
   const reference = entries[0].model.pivot;
   for (const { name, model } of entries) {
     const scene = buildPierScene(model);
-    scene.parts.ground.visible = false;
+    // Without terrain each pier keeps its own 지반고 line; with terrain the shared grid replaces them.
+    scene.parts.ground.visible = terrain === undefined;
     const { x, z } = pierOffset(model.pivot, reference);
     scene.root.position.set(x, 0, z);
     scene.root.userData.pier = name;

src/view/scene.ts

@@ -20,6 +20,9 @@ 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;
+
 const COLORS: Record<PartName, number> = {
   coping: 0xb8c2cc,
   columns: 0x9aa7b4,
@@ -173,17 +176,16 @@ 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),
-  // otherwise a translucent plane at the ground EL, used only to judge soil cover (not a computed input).
+  // 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).
   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 ground = new THREE.PlaneGeometry(60, 60);
-    ground.rotateX(-Math.PI / 2);
-    ground.translate(m.copingL / 2, m.groundEl, 0);
-    add('ground', ground, 0.35);
+    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 root = new THREE.Group();