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/core/pxl8_rng.h

@@ -0,0 +1,20 @@
+#pragma once
+
+#include "pxl8_types.h"
+
+typedef struct pxl8_rng {
+    u32 state;
+} pxl8_rng;
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+void pxl8_rng_seed(pxl8_rng* rng, u32 seed);
+u32 pxl8_rng_next(pxl8_rng* rng);
+f32 pxl8_rng_f32(pxl8_rng* rng);
+i32 pxl8_rng_range(pxl8_rng* rng, i32 min, i32 max);
+
+#ifdef __cplusplus
+}
+#endif