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

demo/mod/menu.fnl

@@ -1,6 +1,10 @@
 (local pxl8 (require :pxl8))
 
 (var paused false)
+(var gui nil)
+
+(fn init []
+  (set gui (pxl8.create_gui)))
 
 (fn show []
   (set paused true)
@@ -12,36 +16,39 @@
   (pxl8.set_relative_mouse_mode true))
 
 (fn toggle []
+  (when (not gui) (init))
   (if paused
       (hide)
       (show)))
 
 (fn update []
-  (let [(mx my) (pxl8.get_mouse_pos)]
-    (pxl8.gui_cursor_move mx my))
+  (when gui
+    (let [(mx my) (pxl8.get_mouse_pos)]
+      (gui:cursor_move mx my))
 
-  (when (pxl8.mouse_pressed 1)
-    (pxl8.gui_cursor_down))
+    (when (pxl8.mouse_pressed 1)
+      (gui:cursor_down))
 
-  (when (pxl8.mouse_released 1)
-    (pxl8.gui_cursor_up)))
+    (when (pxl8.mouse_released 1)
+      (gui:cursor_up))))
 
 (fn draw []
-  (pxl8.gui_begin_frame)
+  (when gui
+    (gui:begin_frame)
 
-  (pxl8.gui_window 200 100 240 140 "pxl8 demo")
+    (pxl8.gui_window 200 100 240 140 "pxl8 demo")
 
-  (when (pxl8.gui_button 1 215 145 210 32 "Resume")
-    (hide))
+    (when (gui:button 1 215 145 210 32 "Resume")
+      (hide))
 
-  (when (pxl8.gui_button 2 215 185 210 32 "Quit")
-    (pxl8.quit))
+    (when (gui:button 2 215 185 210 32 "Quit")
+      (pxl8.quit))
 
-  (if (pxl8.gui_is_hovering)
-      (pxl8.set_cursor :hand)
-      (pxl8.set_cursor :arrow))
+    (if (gui:is_hovering)
+        (pxl8.set_cursor :hand)
+        (pxl8.set_cursor :arrow))
 
-  (pxl8.gui_end_frame))
+    (gui:end_frame)))
 
 {:is-paused (fn [] paused)
  :toggle toggle