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>

tests/terrain.test.ts

@@ -0,0 +1,52 @@
+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, frameMatrix } from '../src/view/scene';
+import { terrainGeometry } from '../src/view/terrain';
+import { loadP1 } from './helpers';
+
+const model = buildPier(loadP1());
+
+function grid(over: Partial<TerrainGrid>): TerrainGrid {
+  return { origin: [259815.676, 179088.094], step: 5, nx: 2, ny: 2, z: [30, 31, 32, 33], ...over };
+}
+
+describe('지형 격자 → 장면 메시 (terrain.ts)', () => {
+  it('a node maps to the same scene point as the pier frame (x = E − pivot E, z = −(N − pivot N))', () => {
+    // A national-grid point on the first stretch: origin + 3·u − 2·nrm (the frame used by the pier scene).
+    const st = model.stretches[0];
+    const q = { e: st.origin.e + 3 * st.u.e - 2 * st.nrm.e, n: st.origin.n + 3 * st.u.n - 2 * st.nrm.n };
+    const expected = new THREE.Vector3(3, 0, -2).applyMatrix4(frameMatrix(model));
+    const g = terrainGeometry(grid({ origin: [q.e, q.n], z: [0, 0, 0, 0] }), model.pivot);
+    const p = new THREE.Vector3().fromBufferAttribute(g.getAttribute('position') as THREE.BufferAttribute, 0);
+    expect(p.x).toBeCloseTo(expected.x, 6);
+    expect(p.y).toBeCloseTo(expected.y, 6);
+    expect(p.z).toBeCloseTo(expected.z, 6);
+  });
+
+  it('the grid node at the pier centre sits at the scene origin, with the sampled EL as height', () => {
+    const centre = grid({ origin: [model.pivot.e - 2.5, model.pivot.n - 2.5], step: 5, nx: 2, ny: 2, z: [28.9, 1, 1, 1] });
+    const g = terrainGeometry(centre, model.pivot);
+    const p = new THREE.Vector3().fromBufferAttribute(g.getAttribute('position') as THREE.BufferAttribute, 0);
+    expect(p.x).toBeCloseTo(-2.5, 9);
+    expect(p.z).toBeCloseTo(2.5, 9);
+    expect(p.y).toBeCloseTo(28.9, 5);                         // stored as Float32
+  });
+
+  it('a cell with a missing corner is not drawn; a full cell gives two triangles', () => {
+    expect(terrainGeometry(grid({ z: [30, null, 32, 33] }), model.pivot).getIndex()!.count).toBe(0);
+    expect(terrainGeometry(grid({}), model.pivot).getIndex()!.count).toBe(6);
+  });
+});
+
+describe('장면에 지형을 넣은 뒤', () => {
+  it('the camera size follows the members, not the extent of the terrain', () => {
+    const small = grid({ origin: [model.pivot.e - 20, model.pivot.n - 20], step: 10, nx: 5, ny: 5, z: new Array(25).fill(28) });
+    const wide = grid({ origin: [model.pivot.e - 200, model.pivot.n - 200], step: 10, nx: 41, ny: 41, z: new Array(41 * 41).fill(28) });
+    const a = buildPierScene(model, small).size;
+    const b = buildPierScene(model, wide).size;
+    expect(b).toBeCloseTo(a, 6);
+    expect(buildPierScene(model, wide).parts.ground.children.length).toBe(1);
+  });
+});