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

Data/expansions/tbc/update_fields.json

@@ -0,0 +1,29 @@
+{
+  "OBJECT_FIELD_ENTRY": 3,
+  "UNIT_FIELD_TARGET_LO": 16,
+  "UNIT_FIELD_TARGET_HI": 17,
+  "UNIT_FIELD_HEALTH": 22,
+  "UNIT_FIELD_POWER1": 23,
+  "UNIT_FIELD_MAXHEALTH": 28,
+  "UNIT_FIELD_MAXPOWER1": 29,
+  "UNIT_FIELD_LEVEL": 34,
+  "UNIT_FIELD_FACTIONTEMPLATE": 35,
+  "UNIT_FIELD_FLAGS": 46,
+  "UNIT_FIELD_FLAGS_2": 47,
+  "UNIT_FIELD_DISPLAYID": 152,
+  "UNIT_FIELD_MOUNTDISPLAYID": 154,
+  "UNIT_NPC_FLAGS": 168,
+  "UNIT_DYNAMIC_FLAGS": 164,
+  "UNIT_END": 234,
+  "PLAYER_FLAGS": 236,
+  "PLAYER_XP": 926,
+  "PLAYER_NEXT_LEVEL_XP": 927,
+  "PLAYER_FIELD_COINAGE": 1441,
+  "PLAYER_QUEST_LOG_START": 244,
+  "PLAYER_FIELD_INV_SLOT_HEAD": 650,
+  "PLAYER_FIELD_PACK_SLOT_1": 696,
+  "PLAYER_SKILL_INFO_START": 928,
+  "PLAYER_EXPLORED_ZONES_START": 1312,
+  "GAMEOBJECT_DISPLAYID": 8,
+  "ITEM_FIELD_STACK_COUNT": 14
+}