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)

src/engine/camera.h

@@ -12,6 +12,8 @@ typedef struct Camera {
     float smoothing;     /* higher = slower follow (0=instant) */
     Vec2  deadzone;      /* half-size of deadzone rect         */
     Vec2  look_ahead;    /* pixels to lead in move direction   */
+    /* Zoom (used by editor; gameplay keeps 1.0) */
+    float zoom;          /* 1.0 = normal, <1 = zoomed out      */
     /* Screen shake */
     float shake_timer;   /* remaining shake time               */
     float shake_intensity; /* current max pixel offset          */