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/terrain-mesh-worker.js

@@ -0,0 +1,13 @@
+import { TerrainState } from './terrain-state.js';
+import { loadLightProperties } from './light-properties.js';
+
+// Geometry never queues behind a whole-view lighting build.
+const state = loadLightProperties().then(properties => new TerrainState(properties));
+self.onmessage = ({data}) => state.then(terrain => {
+  if (data.type === 'sync') {
+    if (terrain.sync(data)) self.postMessage({type:'synced',epoch:data.epoch,version:data.version});
+  } else if (data.type === 'mesh') {
+    const result = terrain.meshLocal(data);
+    self.postMessage(result, result.vertices ? [result.vertices.buffer,result.transparent.buffer] : []);
+  }
+}).catch(error => self.postMessage({...data,type:'error',error:error.message}));