Add tests for terrain mesh generation

tests/geometry/terrain-mesh.test.ts

@@ -0,0 +1,59 @@
+import { describe, expect, it } from "vitest";
+
+import { createTerrain } from "../../src/document/terrains";
+import { buildTerrainDerivedMeshData } from "../../src/geometry/terrain-mesh";
+
+describe("terrain mesh generation", () => {
+  it("chooses the forward diagonal when the opposite-corner height delta is smaller", () => {
+    const terrain = createTerrain({
+      sampleCountX: 2,
+      sampleCountZ: 2,
+      heights: [0, 3, 1, 0]
+    });
+    const derivedMesh = buildTerrainDerivedMeshData(terrain);
+
+    expect(derivedMesh.cellTriangulation).toEqual([
+      {
+        cellX: 0,
+        cellZ: 0,
+        diagonal: "forward"
+      }
+    ]);
+    expect(Array.from(derivedMesh.indices)).toEqual([0, 2, 3, 0, 3, 1]);
+  });
+
+  it("chooses the backward diagonal when that split better matches the local slope", () => {
+    const terrain = createTerrain({
+      sampleCountX: 2,
+      sampleCountZ: 2,
+      heights: [0, 0, 0, 3]
+    });
+    const derivedMesh = buildTerrainDerivedMeshData(terrain);
+
+    expect(derivedMesh.cellTriangulation).toEqual([
+      {
+        cellX: 0,
+        cellZ: 0,
+        diagonal: "backward"
+      }
+    ]);
+    expect(Array.from(derivedMesh.indices)).toEqual([0, 2, 1, 1, 2, 3]);
+  });
+
+  it("derives UVs from world XZ positions instead of triangle-local stretch", () => {
+    const terrain = createTerrain({
+      sampleCountX: 2,
+      sampleCountZ: 2,
+      cellSize: 2,
+      position: {
+        x: 10,
+        y: 4,
+        z: -6
+      },
+      heights: [0, 1, 2, 3]
+    });
+    const derivedMesh = buildTerrainDerivedMeshData(terrain);
+
+    expect(Array.from(derivedMesh.uvs)).toEqual([10, -6, 12, -6, 10, -4, 12, -4]);
+  });
+});