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/block-light-worker.js

@@ -0,0 +1,15 @@
+import { buildBlockLight } from "./block-light.js";
+
+self.onmessage = ({ data: request }) => {
+  try {
+    const start = performance.now();
+    const result = buildBlockLight(new Map(request.blocks), new Map(request.materials), request.bounds);
+    const { min, size, data, sourceCount, blockLevels, skyLevels } = result;
+    self.postMessage({ generation: request.generation,
+      field: { min, size, data, sourceCount, blockLevels, skyLevels },
+      milliseconds: performance.now() - start,
+    }, [data.buffer, blockLevels.buffer, skyLevels.buffer]);
+  } catch (error) {
+    self.postMessage({ generation: request.generation, error: error.message });
+  }
+};