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/audio_callback_handler.cpp

@@ -2,6 +2,8 @@
 #include "core/coordinates.hpp"
 #include "core/logger.hpp"
 #include "rendering/renderer.hpp"
+#include "rendering/animation_controller.hpp"
+#include "pipeline/asset_manager.hpp"
 #include "pipeline/dbc_loader.hpp"
 #include "game/game_handler.hpp"
 #include "audio/audio_coordinator.hpp"
@@ -50,7 +52,7 @@ void AudioCallbackHandler::setupCallbacks() {
             uiManager_->getGameScreen().toastManager().triggerDing(newLevel);
         }
         if (renderer_) {
-            renderer_->triggerLevelUpEffect(renderer_->getCharacterPosition());
+            if (auto* ac = renderer_->getAnimationController()) ac->triggerLevelUpEffect(renderer_->getCharacterPosition());
         }
     });
 
@@ -108,7 +110,7 @@ void AudioCallbackHandler::setupCallbacks() {
         if (entity) {
             glm::vec3 canonical(entity->getX(), entity->getY(), entity->getZ());
             glm::vec3 renderPos = core::coords::canonicalToRender(canonical);
-            renderer_->triggerLevelUpEffect(renderPos);
+            if (auto* ac = renderer_->getAnimationController()) ac->triggerLevelUpEffect(renderPos);
         }
 
         // Show chat message if in group