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

src/ui/inventory_screen.cpp

@@ -7,6 +7,7 @@
 #include "pipeline/asset_manager.hpp"
 #include "pipeline/dbc_loader.hpp"
 #include "pipeline/blp_loader.hpp"
+#include "pipeline/dbc_layout.hpp"
 #include "core/logger.hpp"
 #include <imgui.h>
 #include <SDL2/SDL.h>
@@ -60,7 +61,8 @@ GLuint InventoryScreen::getItemIcon(uint32_t displayInfoId) {
     }
 
     // Field 5 = inventoryIcon_1
-    std::string iconName = displayInfoDbc->getString(static_cast<uint32_t>(recIdx), 5);
+    const auto* dispL = pipeline::getActiveDBCLayout() ? pipeline::getActiveDBCLayout()->getLayout("ItemDisplayInfo") : nullptr;
+    std::string iconName = displayInfoDbc->getString(static_cast<uint32_t>(recIdx), dispL ? (*dispL)["InventoryIcon"] : 5);
     if (iconName.empty()) {
         iconCache_[displayInfoId] = 0;
         return 0;