chore(refactor): god-object decomposition and mega-file splits

Split all mega-files by single-responsibility concern and
partially extracting AudioCoordinator and
OverlaySystem from the Renderer facade. No behavioral changes.

Splits:
- game_handler.cpp (5,247 LOC) → core + callbacks + packets (3 files)
- world_packets.cpp (4,453 LOC) → economy/entity/social/world (4 files)
- game_screen.cpp  (5,786 LOC) → core + frames + hud + minimap (4 files)
- m2_renderer.cpp  (3,343 LOC) → core + instance + particles + render (4 files)
- chat_panel.cpp   (3,140 LOC) → core + commands + utils (3 files)
- entity_spawner.cpp (2,750 LOC) → core + player + processing (3 files)

Extractions:
- AudioCoordinator: include/audio/ + src/audio/ (owned by Renderer)
- OverlaySystem: include/rendering/ + src/rendering/overlay_system.*

CMakeLists.txt: registered all 17 new translation units.
Related handler/callback files: minor include fixups post-split.

src/core/world_loader.cpp

@@ -11,6 +11,7 @@
 #include "core/coordinates.hpp"
 #include "core/logger.hpp"
 #include "rendering/renderer.hpp"
+#include "rendering/animation_controller.hpp"
 #include "rendering/vk_context.hpp"
 #include "rendering/camera.hpp"
 #include "rendering/camera_controller.hpp"
@@ -290,7 +291,7 @@ void WorldLoader::loadOnlineWorldTerrain(uint32_t mapId, float x, float y, float
             if (auto* questMarkers = renderer_->getQuestMarkerRenderer()) {
                 questMarkers->clear();
             }
-            renderer_->clearMount();
+            if (auto* ac = renderer_->getAnimationController()) ac->clearMount();
         }
 
         // Clear application-level instance tracking (after renderer cleanup)
@@ -416,7 +417,7 @@ void WorldLoader::loadOnlineWorldTerrain(uint32_t mapId, float x, float y, float
                     uint32_t oldInst = renderer_->getCharacterInstanceId();
                     if (oldInst > 0) {
                         renderer_->setCharacterFollow(0);
-                        renderer_->clearMount();
+                        if (auto* ac = renderer_->getAnimationController()) ac->clearMount();
                         renderer_->getCharacterRenderer()->removeInstance(oldInst);
                     }
                 }