Add gameplay systems: combat, spells, groups, loot, vendors, and UI

Implement ~70 new protocol opcodes across 5 phases while maintaining
full 3.3.5a private server compatibility:

- Phase 1: Server-aware targeting (CMSG_SET_SELECTION), player/creature
  name queries, CMSG_SET_ACTIVE_MOVER after login
- Phase 2: Auto-attack, melee/spell damage parsing, health/mana/power
  tracking from UPDATE_OBJECT fields, floating combat text
- Phase 3: Spell casting, action bar (12 slots, keys 1-=), cast bar,
  cooldown tracking, aura/buff system with cancellation
- Phase 4: Group invite/accept/decline/leave, party frames UI,
  /invite chat command
- Phase 5: Loot window, NPC gossip dialog, vendor buy/sell interface

Also: disable debug HUD/panels by default, gate 3D rendering to
IN_GAME state only, fix window resize not updating UI positions.

include/rendering/performance_hud.hpp

@@ -72,7 +72,7 @@ private:
      */
     void calculateFPS();
 
-    bool enabled = true;  // Enabled by default, press F1 to toggle
+    bool enabled = false;  // Disabled by default, press F1 to toggle
     Position position = Position::TOP_LEFT;
 
     // Section visibility