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/wotlk/update_fields.json

@@ -0,0 +1,29 @@
+{
+  "OBJECT_FIELD_ENTRY": 3,
+  "UNIT_FIELD_TARGET_LO": 6,
+  "UNIT_FIELD_TARGET_HI": 7,
+  "UNIT_FIELD_HEALTH": 24,
+  "UNIT_FIELD_POWER1": 25,
+  "UNIT_FIELD_MAXHEALTH": 32,
+  "UNIT_FIELD_MAXPOWER1": 33,
+  "UNIT_FIELD_LEVEL": 54,
+  "UNIT_FIELD_FACTIONTEMPLATE": 55,
+  "UNIT_FIELD_FLAGS": 59,
+  "UNIT_FIELD_FLAGS_2": 60,
+  "UNIT_FIELD_DISPLAYID": 67,
+  "UNIT_FIELD_MOUNTDISPLAYID": 69,
+  "UNIT_NPC_FLAGS": 82,
+  "UNIT_DYNAMIC_FLAGS": 147,
+  "UNIT_END": 148,
+  "PLAYER_FLAGS": 150,
+  "PLAYER_XP": 634,
+  "PLAYER_NEXT_LEVEL_XP": 635,
+  "PLAYER_FIELD_COINAGE": 1170,
+  "PLAYER_QUEST_LOG_START": 158,
+  "PLAYER_FIELD_INV_SLOT_HEAD": 324,
+  "PLAYER_FIELD_PACK_SLOT_1": 370,
+  "PLAYER_SKILL_INFO_START": 636,
+  "PLAYER_EXPLORED_ZONES_START": 1041,
+  "GAMEOBJECT_DISPLAYID": 8,
+  "ITEM_FIELD_STACK_COUNT": 14
+}