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/index.ts

@@ -0,0 +1,5 @@
+export * from './types.js';
+export * from './shapes.js';
+export * from './tessellate.js';
+export * from './walls.js';
+export * from './extrude.js';