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_macros.h

@@ -0,0 +1,18 @@
+#pragma once
+
+#include <string.h>
+
+#ifndef pxl8_min
+#define pxl8_min(a, b) ((a) < (b) ? (a) : (b))
+#endif
+
+#ifndef pxl8_max
+#define pxl8_max(a, b) ((a) > (b) ? (a) : (b))
+#endif
+
+#ifndef pxl8_strncpy
+#define pxl8_strncpy(dst, src, size) do { \
+    strncpy((dst), (src), (size) - 1); \
+    (dst)[(size) - 1] = '\0'; \
+} while (0)
+#endif