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/style.css

@@ -18,7 +18,8 @@ body {
   overflow: hidden;
 }
 button,
-input {
+input,
+select {
   font: inherit;
 }
 button {
@@ -33,7 +34,8 @@ button:hover {
   background: #4c5e4a;
 }
 button:focus-visible,
-input:focus-visible {
+input:focus-visible,
+select:focus-visible {
   outline: 2px solid #dfefba;
   outline-offset: 3px;
 }
@@ -47,7 +49,7 @@ button.primary {
   color: #253021;
   font-weight: 650;
 }
-input {
+input, select {
   background: #202821;
   color: #f4f4ef;
   border: 1px solid #576255;
@@ -367,7 +369,7 @@ footer {
   gap: 6px;
   margin-bottom: 20px;
 }
-.inline input {
+.inline input, .inline select {
   min-width: 0;
 }
 .inline button {
@@ -595,3 +597,6 @@ body.locked .topbar:hover {
     inset: 25% 14px auto;
   }
 }
+
+/* Keep the live readout beside menus, while the world remains interactive. */
+#diagnostics { left: 24px; right: auto; }