feat(mesh): add tile mesh generation package

Add @tts/mesh, a package for generating extruded tile meshes from 2D
shapes. It provides shape generators (rect, hex, circle, rounded rect,
frame), earcut-based tessellation of top/bottom faces with UVs, and
outward-normal wall generation, combined into raw BufferGeometry-ready
typed arrays.

packages/mesh/src/walls.test.ts

@@ -0,0 +1,38 @@
+import { describe, expect, it } from 'vitest';
+import { wallFaces } from './walls.js';
+import { rectShape } from './shapes.js';
+
+describe('wallFaces', () => {
+  it('builds a quad strip around a rectangle', () => {
+    const wall = wallFaces(rectShape(2, 2), 1);
+    // 4 outline points * 2 vertices each.
+    expect(wall.positions.length / 3).toBe(8);
+    expect(wall.indices.length).toBe(4 * 6); // 4 quads * 2 triangles
+  });
+
+  it('has outward-facing normals', () => {
+    const wall = wallFaces(rectShape(2, 2), 1);
+    // Bottom edge (from (-1,-1) to (1,-1)) normal should point -Y.
+    // First two vertices belong to the first outline point (-1,-1).
+    const n = wall.normals.slice(0, 3);
+    expect(n[0]).toBeCloseTo(0, 5);
+    expect(n[1]).toBeCloseTo(-1, 5);
+    expect(n[2]).toBeCloseTo(0, 5);
+  });
+
+  it('spans from z=0 to z=height', () => {
+    const wall = wallFaces(rectShape(2, 2), 3);
+    // First vertex is bottom (z=0), second is top (z=3).
+    expect(wall.positions[2]).toBe(0);
+    expect(wall.positions[5]).toBe(3);
+  });
+
+  it('maps u across the perimeter and v up the height', () => {
+    const wall = wallFaces(rectShape(2, 2), 2);
+    // u starts at 0 for the first outline point.
+    expect(wall.uvs[0]).toBeCloseTo(0, 5);
+    // v goes 0 -> 1 from bottom to top.
+    expect(wall.uvs[1]).toBeCloseTo(0, 5);
+    expect(wall.uvs[3]).toBeCloseTo(1, 5);
+  });
+});