refactor: add why-comments to zone tiles, audio cache, socket buffer

- zone_manager: document tile-to-zone key encoding (tileX * 100 + tileY,
  safe because tileY < 64 < 100) and explain that ranges are empirically
  derived from the retail WoW map grid
- audio_engine: expand sample rate comment — miniaudio defaults to
  device rate causing pitch distortion if not set explicitly; name
  kMaxCachedSounds constant with memory budget explanation
- tcp_socket: add why-comment on 4 KB recv buffer sizing — covers
  typical 20-500 byte packets and worst-case ~2 KB UPDATE_OBJECT

src/network/tcp_socket.cpp

@@ -139,6 +139,9 @@ void TCPSocket::update() {
     bool sawClose = false;
     bool receivedAny = false;
     for (;;) {
+        // 4 KB per recv() call — large enough for any single game packet while keeping
+        // stack usage reasonable. Typical WoW packets are 20-500 bytes; UPDATE_OBJECT
+        // can reach ~2 KB in crowded zones.
         uint8_t buffer[4096];
         ssize_t received = net::portableRecv(sockfd, buffer, sizeof(buffer));