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/transport_callback_handler.cpp

@@ -4,6 +4,7 @@
 #include "core/coordinates.hpp"
 #include "core/logger.hpp"
 #include "rendering/renderer.hpp"
+#include "rendering/animation_controller.hpp"
 #include "rendering/character_renderer.hpp"
 #include "rendering/camera_controller.hpp"
 #include "rendering/terrain_manager.hpp"
@@ -37,7 +38,7 @@ void TransportCallbackHandler::setupCallbacks() {
                 entitySpawner_.clearMountState();
             }
             entitySpawner_.setMountDisplayId(0);
-            renderer_.clearMount();
+            if (auto* ac = renderer_.getAnimationController()) ac->clearMount();
             LOG_INFO("Dismounted");
             return;
         }
@@ -106,7 +107,7 @@ void TransportCallbackHandler::setupCallbacks() {
             renderer_.getCameraController()->setFacingYaw(yawDegrees);
             renderer_.setCharacterYaw(yawDegrees);
             // Set mount pitch and roll for realistic flight animation
-            renderer_.setMountPitchRoll(pitch, roll);
+            if (auto* ac = renderer_.getAnimationController()) ac->setMountPitchRoll(pitch, roll);
         }
     });