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-worker.js

@@ -0,0 +1,27 @@
+import { TerrainState } from "./terrain-state.js";
+import { loadLightProperties } from "./light-properties.js";
+
+const state = loadLightProperties().then(properties => new TerrainState(properties));
+let timer = null;
+const send = message => {
+  const buffers = message.type === "light"
+    ? [message.field.data.buffer, message.field.blockLevels.buffer, message.field.skyLevels.buffer]
+    : message.vertices ? [message.vertices.buffer, message.transparent.buffer] : [];
+  self.postMessage(message, buffers);
+};
+self.onmessage = ({ data }) => {
+  state.then(terrain => {
+    if (data.type === "sync") {
+      if (!terrain.sync(data)) return;
+      clearTimeout(timer);
+      timer = setTimeout(() => {
+        timer = null;
+        try { send(terrain.light({dirty:!data.independentMeshes})); }
+        catch (error) { send({ type: "error", epoch: terrain.epoch, version: terrain.version, error: error.message }); }
+      }, data.independentMeshes ? 225 : 0);
+    } else if (data.type === "mesh") {
+      try { send(terrain.mesh(data)); }
+      catch (error) { send({ ...data, error: error.message }); }
+    }
+  }).catch(error => send({ type: "error", epoch: data.epoch, version: data.version, error: error.message }));
+};