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

CMakeLists.txt

@@ -95,6 +95,9 @@ set(WOWEE_SOURCES
     src/auth/rc4.cpp
 
     # Game
+    src/game/expansion_profile.cpp
+    src/game/opcode_table.cpp
+    src/game/update_field_table.cpp
     src/game/game_handler.cpp
     src/game/warden_crypto.cpp
     src/game/warden_module.cpp
@@ -105,6 +108,8 @@ set(WOWEE_SOURCES
     src/game/entity.cpp
     src/game/opcodes.cpp
     src/game/world_packets.cpp
+    src/game/packet_parsers_tbc.cpp
+    src/game/packet_parsers_classic.cpp
     src/game/character.cpp
     src/game/zone_manager.cpp
     src/game/inventory.cpp
@@ -131,6 +136,8 @@ set(WOWEE_SOURCES
     src/pipeline/m2_loader.cpp
     src/pipeline/wmo_loader.cpp
     src/pipeline/adt_loader.cpp
+    src/pipeline/dbc_layout.cpp
+    src/pipeline/hd_pack_manager.cpp
     src/pipeline/terrain_mesh.cpp
 
     # Rendering