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/tilemap.h

@@ -46,6 +46,7 @@ typedef struct Tilemap {
     bool         has_bg_color;      /* true if BG_COLOR was set                */
     char         parallax_far_path[ASSET_PATH_MAX];  /* far bg image path     */
     char         parallax_near_path[ASSET_PATH_MAX]; /* near bg image path    */
+    int          parallax_style;    /* procedural bg style (0=default)         */
     EntitySpawn  entity_spawns[MAX_ENTITY_SPAWNS];
     int          entity_spawn_count;
 } Tilemap;