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.

include/rendering/overlay_system.hpp

@@ -0,0 +1,73 @@
+#pragma once
+
+#include <glm/vec3.hpp>
+#include <glm/vec4.hpp>
+#include <glm/mat4x4.hpp>
+#include <vulkan/vulkan.h>
+#include <vk_mem_alloc.h>
+#include <functional>
+#include <optional>
+
+namespace wowee {
+namespace rendering {
+
+class VkContext;
+
+/// Manages selection circle and fullscreen overlay Vulkan pipelines.
+/// Extracted from Renderer to isolate overlay rendering resources.
+class OverlaySystem {
+public:
+    /// Height query callable: returns floor height at (x, y) or (x, y, probeZ).
+    using HeightQuery2D = std::function<std::optional<float>(float x, float y)>;
+    using HeightQuery3D = std::function<std::optional<float>(float x, float y, float probeZ)>;
+
+    explicit OverlaySystem(VkContext* ctx);
+    ~OverlaySystem();
+
+    OverlaySystem(const OverlaySystem&) = delete;
+    OverlaySystem& operator=(const OverlaySystem&) = delete;
+
+    // Selection circle
+    void setSelectionCircle(const glm::vec3& pos, float radius, const glm::vec3& color);
+    void clearSelectionCircle();
+    void renderSelectionCircle(const glm::mat4& view, const glm::mat4& projection,
+                               VkCommandBuffer cmd,
+                               HeightQuery2D terrainHeight,
+                               HeightQuery3D wmoHeight,
+                               HeightQuery3D m2Height);
+
+    // Fullscreen color overlay (underwater tint, etc.)
+    void renderOverlay(const glm::vec4& color, VkCommandBuffer cmd);
+
+    /// Destroy all Vulkan resources (called before VkContext teardown).
+    void cleanup();
+
+    /// Recreate pipelines after swapchain resize / MSAA change.
+    void recreatePipelines();
+
+private:
+    void initSelectionCircle();
+    void initOverlayPipeline();
+
+    VkContext* vkCtx_ = nullptr;
+
+    // Selection circle resources
+    VkPipeline selCirclePipeline_ = VK_NULL_HANDLE;
+    VkPipelineLayout selCirclePipelineLayout_ = VK_NULL_HANDLE;
+    ::VkBuffer selCircleVertBuf_ = VK_NULL_HANDLE;
+    VmaAllocation selCircleVertAlloc_ = VK_NULL_HANDLE;
+    ::VkBuffer selCircleIdxBuf_ = VK_NULL_HANDLE;
+    VmaAllocation selCircleIdxAlloc_ = VK_NULL_HANDLE;
+    int selCircleVertCount_ = 0;
+    glm::vec3 selCirclePos_{0.0f};
+    glm::vec3 selCircleColor_{1.0f, 0.0f, 0.0f};
+    float selCircleRadius_ = 1.5f;
+    bool selCircleVisible_ = false;
+
+    // Fullscreen overlay resources
+    VkPipeline overlayPipeline_ = VK_NULL_HANDLE;
+    VkPipelineLayout overlayPipelineLayout_ = VK_NULL_HANDLE;
+};
+
+} // namespace rendering
+} // namespace wowee