refactor: reorganize pxl8 into client/src/ module structure

- core/: main entry, types, logging, I/O, RNG
- asset/: ase loader, cart, save, embed
- gfx/: graphics, animation, atlas, fonts, tilemap, transitions
- sfx/: audio
- script/: lua/fennel runtime, REPL
- hal/: platform abstraction (SDL3)
- world/: BSP, world, procedural gen
- math/: math utilities
- game/: GUI, replay
- lua/: Lua API modules

client/src/hal/pxl8_hal.h

@@ -0,0 +1,23 @@
+#pragma once
+
+#include "pxl8_types.h"
+
+typedef struct pxl8_hal {
+    void* (*create)(i32 render_w, i32 render_h,
+                    const char* title, i32 win_w, i32 win_h);
+    void (*destroy)(void* platform_data);
+    u64 (*get_ticks)(void);
+    void (*center_cursor)(void* platform_data);
+    void (*present)(void* platform_data);
+    void (*set_cursor)(void* platform_data, u32 cursor);
+    void (*set_relative_mouse_mode)(void* platform_data, bool enabled);
+    void (*upload_texture)(void* platform_data, const u8* pixels, u32 w, u32 h,
+                           const u32* palette, u32 bpp);
+
+    void* (*audio_create)(i32 sample_rate, i32 channels);
+    void (*audio_destroy)(void* audio_handle);
+    void (*audio_start)(void* audio_handle);
+    void (*audio_stop)(void* audio_handle);
+    bool (*upload_audio)(void* audio_handle, const f32* stereo_samples, i32 sample_count);
+    i32 (*audio_queued)(void* audio_handle);
+} pxl8_hal;