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_sdl3.h

@@ -0,0 +1,5 @@
+#pragma once
+
+#include "pxl8_hal.h"
+
+extern const pxl8_hal pxl8_hal_sdl3;