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/shadow-frame.js

@@ -0,0 +1,25 @@
+/** Directional orthographic projection, snapped in light space to avoid crawling shadows. */
+export function shadowFrame(eye, sun, resolution = 2048, radius = 48) {
+  const length = Math.hypot(...sun),
+    z = sun.map((v) => v / length),
+    horizontal = Math.hypot(z[0], z[2]),
+    x = horizontal > 1e-6 ? [z[2] / horizontal, 0, -z[0] / horizontal] : [1, 0, 0],
+    y = [z[1] * x[2] - z[2] * x[1], z[2] * x[0] - z[0] * x[2], z[0] * x[1] - z[1] * x[0]],
+    dot = (a, b) => a.reduce((sum, v, i) => sum + v * b[i], 0),
+    texel = 2 * radius / resolution,
+    sx = Math.round(dot(x, eye) / texel) * texel,
+    sy = Math.round(dot(y, eye) / texel) * texel,
+    sz = dot(z, eye),
+    depth = radius * 2,
+    center = eye.map((_, i) => x[i] * sx + y[i] * sy + z[i] * sz);
+  return {
+    center,
+    radius,
+    matrix: new Float32Array([
+      x[0] / radius, y[0] / radius, -z[0] / depth, 0,
+      x[1] / radius, y[1] / radius, -z[1] / depth, 0,
+      x[2] / radius, y[2] / radius, -z[2] / depth, 0,
+      -sx / radius, -sy / radius, sz / depth, 1,
+    ]),
+  };
+}