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/light-changes.js

@@ -0,0 +1,28 @@
+/** Compare only visible mesh neighborhoods; unchanged retained sections keep their buffers. */
+export function changedLightSections(previous, next, sections) {
+  if (!previous) return [...sections];
+  const index = (field, x, y, z) => {
+    x -= field.min[0]; y -= field.min[1]; z -= field.min[2];
+    if (x < 0 || y < 0 || z < 0 || x >= field.size[0] || y >= field.size[1] || z >= field.size[2]) return -1;
+    return x + field.size[0] * (y + field.size[1] * z);
+  };
+  const changed = [];
+  for (const id of sections) {
+    const base = id.split(",").map(v => Number(v) * 16);
+    let dirty = false;
+    outer: for (let z = base[2] - 1; z <= base[2] + 16; z++)
+      for (let y = base[1] - 1; y <= base[1] + 16; y++)
+        for (let x = base[0] - 1; x <= base[0] + 16; x++) {
+          const a = index(previous, x, y, z), b = index(next, x, y, z);
+          if (a < 0 && b < 0) continue;
+          if (a < 0 || b < 0 || previous.skyLevels[a] !== next.skyLevels[b] ||
+              previous.data[a * 4] !== next.data[b * 4] ||
+              previous.data[a * 4 + 1] !== next.data[b * 4 + 1] ||
+              previous.data[a * 4 + 2] !== next.data[b * 4 + 2]) {
+            dirty = true; break outer;
+          }
+        }
+    if (dirty) changed.push(id);
+  }
+  return changed;
+}