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/parallax.h

@@ -37,6 +37,17 @@ void parallax_generate_stars(Parallax *p, SDL_Renderer *renderer);
 /* Generate procedural nebula/dust texture (near layer) */
 void parallax_generate_nebula(Parallax *p, SDL_Renderer *renderer);
 
+/* Themed parallax generation styles */
+typedef enum ParallaxStyle {
+    PARALLAX_STYLE_DEFAULT,     /* generic space (same as stars+nebula)     */
+    PARALLAX_STYLE_ALIEN_SKY,   /* alien planet surface: dusty, hazy       */
+    PARALLAX_STYLE_INTERIOR,    /* indoor base: panels, pipes, structural  */
+    PARALLAX_STYLE_DEEP_SPACE,  /* space station windows: vivid stars      */
+} ParallaxStyle;
+
+/* Generate both layers with a unified style */
+void parallax_generate_themed(Parallax *p, SDL_Renderer *renderer, ParallaxStyle style);
+
 /* Render both layers (call before tile/entity rendering) */
 void parallax_render(const Parallax *p, const Camera *cam, SDL_Renderer *renderer);