Hide post-login world hitch behind loading screen

- keep character-selection state until world entry load/finalize completes
- move expensive post-load setup (test transport + creature callback prep) before loading screen shutdown
- add bounded world warmup pass under loading screen to drain initial network/spawn backlog
- start intro camera pan after warmup so rotation begins when gameplay becomes visible
- guard test transport setup so it runs once per session
- add per-update world socket parse budget to prevent single-frame packet-drain stalls

This reduces visible 3-4s stutter after login by shifting startup work behind the loading screen and time-slicing packet processing.

src/network/world_socket.cpp

@@ -10,6 +10,7 @@
 
 namespace {
 constexpr size_t kMaxReceiveBufferBytes = 8 * 1024 * 1024;
+constexpr int kMaxParsedPacketsPerUpdate = 220;
 
 inline bool isLoginPipelineSmsg(uint16_t opcode) {
     switch (opcode) {
@@ -323,7 +324,8 @@ void WorldSocket::update() {
 
 void WorldSocket::tryParsePackets() {
     // World server packets have 4-byte incoming header: size(2) + opcode(2)
-    while (receiveBuffer.size() >= 4) {
+    int parsedThisTick = 0;
+    while (receiveBuffer.size() >= 4 && parsedThisTick < kMaxParsedPacketsPerUpdate) {
         uint8_t rawHeader[4] = {0, 0, 0, 0};
         std::memcpy(rawHeader, receiveBuffer.data(), 4);
 
@@ -417,6 +419,12 @@ void WorldSocket::tryParsePackets() {
         if (packetCallback) {
             packetCallback(packet);
         }
+        ++parsedThisTick;
+    }
+
+    if (parsedThisTick >= kMaxParsedPacketsPerUpdate && receiveBuffer.size() >= 4) {
+        LOG_DEBUG("World socket parse budget reached (", parsedThisTick,
+                 " packets); deferring remaining buffered data=", receiveBuffer.size(), " bytes");
     }
 }