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/game/pxl8_gui.h

@@ -0,0 +1,38 @@
+#pragma once
+
+#include "pxl8_gfx.h"
+#include "pxl8_types.h"
+
+typedef struct {
+    i32 cursor_x;
+    i32 cursor_y;
+    bool cursor_down;
+    bool cursor_clicked;
+    u32 hot_id;
+    u32 active_id;
+} pxl8_gui_state;
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+pxl8_gui_state* pxl8_gui_state_create(void);
+void pxl8_gui_state_destroy(pxl8_gui_state* state);
+
+void pxl8_gui_get_cursor_pos(const pxl8_gui_state* state, i32* x, i32* y);
+bool pxl8_gui_is_hovering(const pxl8_gui_state* state);
+
+void pxl8_gui_begin_frame(pxl8_gui_state* state);
+void pxl8_gui_end_frame(pxl8_gui_state* state);
+
+void pxl8_gui_cursor_down(pxl8_gui_state* state);
+void pxl8_gui_cursor_move(pxl8_gui_state* state, i32 x, i32 y);
+void pxl8_gui_cursor_up(pxl8_gui_state* state);
+
+bool pxl8_gui_button(pxl8_gui_state* state, pxl8_gfx* gfx, u32 id, i32 x, i32 y, i32 w, i32 h, const char* label);
+void pxl8_gui_label(pxl8_gfx* gfx, i32 x, i32 y, const char* text, u8 color);
+void pxl8_gui_window(pxl8_gfx* gfx, i32 x, i32 y, i32 w, i32 h, const char* title);
+
+#ifdef __cplusplus
+}
+#endif