Add in-game level editor with auto-discovered tile/entity palettes

Implements a full level editor that runs inside the game engine as an
alternative mode, accessible via --edit flag or E key during gameplay.
The editor auto-discovers available tiles from the tileset texture and
entities from a new central registry, so adding new game content
automatically appears in the editor without any editor-specific changes.

Editor features: tile painting (pencil/eraser/flood fill) across 3
layers, entity placement with drag-to-move, player spawn point tool,
camera pan/zoom, grid overlay, .lvl save/load, map resize, and test
play (P to play, ESC to return to editor).

Supporting changes:
- Entity registry centralizes spawn functions (replaces strcmp chain)
- Mouse input + raw keyboard access added to input system
- Camera zoom support for editor overview
- Zoom-aware rendering in tilemap, renderer, and sprite systems
- Powerup and drone sprites/animations wired up (were defined but unused)
- Bitmap font renderer for editor UI (4x6 pixel glyphs, no dependencies)

assets/levels/level01.lvl

@@ -16,6 +16,13 @@ ENTITY grunt 9  16
 ENTITY flyer 20 10
 ENTITY flyer 30 8
 
+# Hazards
+ENTITY turret 33 4
+ENTITY flame_vent 18 19
+ENTITY force_field 35 17
+ENTITY platform 12 11
+ENTITY platform_v 27 12
+
 # Tile definitions: ID TEX_X TEX_Y FLAGS
 # Flag 1 = SOLID, Flag 2 = PLATFORM
 TILEDEF 1 0 0 1