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/light-changes.test.js

@@ -0,0 +1,69 @@
+import test from "node:test";
+import assert from "node:assert/strict";
+import { changedLightSections } from "../light-changes.js";
+
+function field(min = [-2, -2, -2], size = [54, 36, 36]) {
+  const count = size.reduce((product, value) => product * value, 1);
+  return {
+    min,
+    size,
+    data: new Uint8Array(count * 4),
+    skyLevels: new Uint8Array(count).fill(15),
+  };
+}
+const index = (field, [x, y, z]) =>
+  x -
+  field.min[0] +
+  field.size[0] * (y - field.min[1] + field.size[1] * (z - field.min[2]));
+
+test("initial lighting marks current sections, while equal fields retain every mesh", () => {
+  const sections = new Map([
+    ["0,0,0", { vao: "first" }],
+    ["1,0,0", { vao: "retained" }],
+  ]);
+  assert.deepEqual(changedLightSections(null, field(), sections.keys()), [
+    ...sections.keys(),
+  ]);
+  const before = [...sections.values()];
+  assert.deepEqual(changedLightSections(field(), field(), sections.keys()), []);
+  assert.deepEqual([...sections.values()], before);
+});
+
+test("a corner light delta invalidates all eight sharing neighborhoods but no distant section", () => {
+  const previous = field(),
+    next = field();
+  next.skyLevels[index(next, [15, 15, 15])] = 14;
+  const neighbors = [];
+  for (let x = 0; x <= 1; x++)
+    for (let y = 0; y <= 1; y++)
+      for (let z = 0; z <= 1; z++) neighbors.push(`${x},${y},${z}`);
+  assert.deepEqual(
+    changedLightSections(previous, next, [...neighbors, "2,0,0"]),
+    neighbors,
+  );
+});
+
+test("a tint-only change crosses the section boundary and preserves distant retained sections", () => {
+  const previous = field(),
+    next = field();
+  next.data[index(next, [16, 4, 4]) * 4 + 2] = 180;
+  const sections = ["0,0,0", "1,0,0", "2,0,0"];
+  assert.deepEqual(changedLightSections(previous, next, sections), [
+    "0,0,0",
+    "1,0,0",
+  ]);
+});
+
+test("moving field bounds compares world coordinates and only invalidates lost coverage", () => {
+  const previous = field();
+  const shifted = field([-1, -2, -2], [55, 36, 36]);
+  assert.deepEqual(
+    changedLightSections(previous, shifted, ["0,0,0", "1,0,0"]),
+    [],
+  );
+  const clipped = field([0, -2, -2], [54, 36, 36]);
+  assert.deepEqual(
+    changedLightSections(previous, clipped, ["0,0,0", "1,0,0"]),
+    ["0,0,0"],
+  );
+});