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>

tests/all-piers.test.ts

@@ -31,11 +31,15 @@ describe('buildAllPiers', () => {
     expect(b.max.distanceTo(a.max)).toBeLessThan(1e-4);
   });
 
-  it('hides each pier ground plane and keys scenes by name', () => {
-    const all = buildAllPiers([{ name: 'P1', model }]);
-    expect([...all.scenes.keys()]).toEqual(['P1']);
-    expect(all.scenes.get('P1')!.parts.ground.visible).toBe(false);
-    expect(all.terrain).toBeNull();
-    expect(all.view.size).toBeGreaterThan(0);
+  it('keeps each pier 지반고 line without terrain, and hides it when the shared terrain is drawn', () => {
+    const without = buildAllPiers([{ name: 'P1', model }]);
+    expect([...without.scenes.keys()]).toEqual(['P1']);
+    expect(without.scenes.get('P1')!.parts.ground.visible).toBe(true);
+    expect(without.terrain).toBeNull();
+    expect(without.view.size).toBeGreaterThan(0);
+
+    const withTerrain = buildAllPiers([{ name: 'P1', model }], { origin: [259815.676, 179088.094], step: 5, nx: 2, ny: 2, z: [30, 31, 32, 33] });
+    expect(withTerrain.scenes.get('P1')!.parts.ground.visible).toBe(false);
+    expect(withTerrain.terrain).not.toBeNull();
   });
 });

tests/ground-marker.test.ts

@@ -0,0 +1,38 @@
+import * as THREE from 'three';
+import { describe, expect, it } from 'vitest';
+import { buildPier } from '../src/core/geometry';
+import type { TerrainGrid } from '../src/core/ground-model';
+import { buildPierScene } from '../src/view/scene';
+import { loadP1 } from './helpers';
+
+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', () => {
+    const scene = buildPierScene(model);
+    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.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', () => {
+    const scene = buildPierScene(model);
+    expect(scene.size).toBeLessThan(60);
+  });
+
+  it('does not draw the line when the terrain grid is present (the terrain replaces it)', () => {
+    const scene = buildPierScene(model, terrain);
+    expect(scene.parts.ground.children.length).toBe(1);
+    const mesh = scene.parts.ground.children[0] as THREE.Mesh;
+    expect(mesh.geometry.type).toBe('BufferGeometry');
+  });
+});