Expand voxel gameplay, lighting, full-height streaming and world imports

Add shared Rust/WASM physics, worker meshing and diagnostics, 64-chunk full-height streaming, atlas texture support, and baseline world import. Document the current implementation and include the supplied in-game lobby screenshot.

client/local-light-bounds.js

@@ -0,0 +1,10 @@
+// Fifteen light levels plus the halo sampled by faces and partial models.
+export const LOCAL_LIGHT_HALO = 18;
+export function localLightBounds(id, view) {
+  if (!view) return null;
+  const [x,,z] = id.split(',').map(value=>Math.floor(Number(value)/2)*32);
+  return {
+    min:[Math.max(view.min[0],x-LOCAL_LIGHT_HALO),view.min[1],Math.max(view.min[2],z-LOCAL_LIGHT_HALO)],
+    max:[Math.min(view.max[0],x+31+LOCAL_LIGHT_HALO),view.max[1],Math.min(view.max[2],z+31+LOCAL_LIGHT_HALO)],
+  };
+}