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/section-block-map.js

@@ -0,0 +1,68 @@
+const sectionKey = (key) =>
+  key
+    .split(",")
+    .map((value) => Math.floor(Number(value) / 16))
+    .join(",");
+
+/** Block positions indexed by their 16³ section without copying block values. */
+export class SectionBlockMap extends Map {
+  #sections = new Map();
+
+  constructor(entries) {
+    super();
+    if (entries != null)
+      for (const [key, value] of entries) this.set(key, value);
+  }
+
+  set(key, value) {
+    if (!super.has(key)) this.#index(key, sectionKey(key));
+    return super.set(key, value);
+  }
+
+  /** Insert using a section ID already validated by the caller. */
+  setInSection(key, value, section) {
+    if (!super.has(key)) this.#index(key, section);
+    return super.set(key, value);
+  }
+
+  #index(key, section) {
+    let keys = this.#sections.get(section);
+    if (!keys) this.#sections.set(section, (keys = new Set()));
+    keys.add(key);
+  }
+
+  delete(key) {
+    if (!super.delete(key)) return false;
+    const section = sectionKey(key),
+      keys = this.#sections.get(section);
+    keys.delete(key);
+    if (!keys.size) this.#sections.delete(section);
+    return true;
+  }
+
+  /** Remove every stored block in a section and return the number removed. */
+  deleteSection(section) {
+    const keys = this.#sections.get(section);
+    if (!keys) return 0;
+    const count = keys.size;
+    for (const key of keys) super.delete(key);
+    keys.clear();
+    this.#sections.delete(section);
+    return count;
+  }
+
+  clear() {
+    super.clear();
+    this.#sections.clear();
+  }
+
+  /** Live key iterator; an absent or empty section contains no stored blocks. */
+  keysInSection(section) {
+    return this.#sections.get(section)?.keys() ?? [][Symbol.iterator]();
+  }
+
+  /** Sections containing stored blocks; empty loaded sections have no entries. */
+  loadedSectionKeys() {
+    return this.#sections.keys();
+  }
+}