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/tests/renderer-streaming.test.js

@@ -0,0 +1,47 @@
+import test from "node:test";
+import assert from "node:assert/strict";
+import { Renderer } from "../renderer.js";
+
+function meshFixture() {
+  const renderer = Object.create(Renderer.prototype);
+  renderer.sections = new Map([
+    ["-2,0,0", { name: "leaving" }],
+    ["-1,0,0", { name: "border" }],
+    ["0,0,0", { name: "retained" }],
+    ["1,0,0", { name: "far" }],
+  ]);
+  renderer.dirty = new Set();
+  renderer.disposed = [];
+  renderer.disposeMesh = (mesh) => renderer.disposed.push(mesh);
+  return renderer;
+}
+
+test("stream unload disposes only departing meshes and keeps visible mesh identities", () => {
+  const renderer = meshFixture();
+  const before = new Map(renderer.sections);
+  renderer.dirty.add("-2,0,0");
+  renderer.unloadSections([[-2, 0, 0]]);
+  assert.deepEqual(renderer.disposed, [before.get("-2,0,0")]);
+  for (const id of ["-1,0,0", "0,0,0", "1,0,0"])
+    assert.equal(renderer.sections.get(id), before.get(id));
+  assert.deepEqual([...renderer.dirty], ["-1,0,0"]);
+});
+
+test("incoming cells queue their section and boundaries without clearing existing geometry", () => {
+  const renderer = meshFixture();
+  const before = [...renderer.sections.values()];
+  renderer.change([{ pos: [32, 5, 5], block: 7 }]);
+  assert.deepEqual([...renderer.sections.values()], before);
+  assert.equal(renderer.disposed.length, 0);
+  assert.deepEqual([...renderer.dirty].sort(), ["1,0,0", "2,0,0"]);
+});
+
+test("unloading adjacent sections does not queue either for reconstruction", () => {
+  const renderer = meshFixture();
+  renderer.change([{ pos: [-17, 0, 0], block: 0 }]);
+  renderer.unloadSections([[-2, 0, 0], [-1, 0, 0]]);
+  assert.equal(renderer.dirty.has("-2,0,0"), false);
+  assert.equal(renderer.dirty.has("-1,0,0"), false);
+  assert.equal(renderer.dirty.has("0,0,0"), true);
+  assert.equal(renderer.sections.size, 2);
+});