Add multi-expansion support with data-driven protocol layer

Replace hardcoded WotLK protocol constants with a data-driven architecture
supporting Classic 1.12.1, TBC 2.4.3, and WotLK 3.3.5a. Each expansion
has JSON profiles for opcodes, update fields, and DBC layouts, plus C++
polymorphic packet parsers for binary format differences (movement flags,
speed fields, transport data, spline format, char enum layout).

Key components:
- ExpansionRegistry: scans Data/expansions/*/expansion.json at startup
- OpcodeTable: logical enum <-> wire values loaded from JSON
- UpdateFieldTable: field indices loaded from JSON per expansion
- DBCLayout: schema-driven DBC field lookups replacing magic numbers
- PacketParsers: WotLK/TBC/Classic parsers with correct flag positions
- Multi-manifest AssetManager: layered manifests with priority ordering
- HDPackManager: overlay texture packs with expansion compatibility
- Auth screen expansion picker replacing hardcoded version dropdown

include/ui/auth_screen.hpp

@@ -3,6 +3,7 @@
 #include "auth/auth_handler.hpp"
 #include "rendering/video_player.hpp"
 #include <string>
+#include <vector>
 #include <functional>
 
 namespace wowee { namespace ui {
@@ -46,7 +47,7 @@ private:
     char username[256] = "";
     char password[256] = "";
     int port = 3724;
-    int compatibilityMode = 0;  // 0 = 3.3.5a
+    int expansionIndex = 0;     // Index into expansion registry profiles
     bool authenticating = false;
     bool showPassword = false;