pickups, native build, enemy/bullet/stage overhaul

- Add pickup system (bomb, spread, rapid, shield) with new sprites
- Replace Docker build with native SGDK compile via m68k-elf-gcc
- Rework enemy spawning, homing math, boss HP/number globals
- Expand chrome: score popups, minimap, pause/game over improvements
- Overhaul stage generation with threat-point system
- Add explosion sprites, shield sprite, powerup sprite
- Add tools/ for sprite downscaling utilities

.gitignore

@@ -17,4 +17,5 @@ dist/
 **/**.psd
 *.pdf
 **/**.pdf
-docs/
+docs/
+sgdk/