feat(ground): draw the terrain grid around the piers in the 3D view

- ground-model.ts: read schema 2 terrain_grid (origin, step, nx, ny, z),
  checked the same way as the stored project form; schema 1 is still read.
- view/terrain.ts: the grid as a mesh in pier-scene coordinates
  (x = E − pivot E, z = −(N − pivot N), y = EL), same as the pier frame.
  Cells with a missing corner are not drawn.
- scene.ts: draws the terrain in place of the 60 m plane when the model has a
  grid; the camera fits the members, not the terrain.

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

src/main.ts

@@ -317,7 +317,7 @@ function setupScene(): void {
     rebar3d.group.visible = state.stage === 'rebar';
     rebar3d.setColorMode(state.rebarColor);
     world.add(rebar3d.group);
-    const scene = buildPierScene(pier.model);
+    const scene = buildPierScene(pier.model, groundModel?.terrainGrid);
     for (const key of Object.keys(state.visible) as PartName[]) {
       scene.parts[key].visible = state.visible[key];
     }
@@ -1512,6 +1512,7 @@ function renderGroundModel(box: HTMLDivElement, pier: Pier): void {
     try {
       groundModel = parseGroundModel(await file.text());
       setStatus(`${file.name}에서 교각 ${groundModel.piers.length}개의 지반 모델 값을 불러왔습니다. 적용은 교각별로 누르세요.`);
+      setupScene();
       renderAll();
     } catch (err) {
       setError(`지반 모델을 읽지 못했습니다: ${(err as Error).message}`);