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/input.h

@@ -27,4 +27,27 @@ void input_shutdown(void);
 /* Returns true if SDL_QUIT was received */
 bool input_quit_requested(void);
 
+/* ── Mouse state ────────────────────────────────── */
+typedef enum MouseButton {
+    MOUSE_LEFT   = 0,
+    MOUSE_MIDDLE = 1,
+    MOUSE_RIGHT  = 2,
+    MOUSE_BUTTON_COUNT
+} MouseButton;
+
+/* Mouse position in logical (game) coordinates */
+void input_mouse_pos(int *x, int *y);
+
+/* Mouse button queries (same semantics as keyboard) */
+bool input_mouse_pressed(MouseButton btn);
+bool input_mouse_held(MouseButton btn);
+bool input_mouse_released(MouseButton btn);
+
+/* Scroll wheel delta since last poll (positive = up) */
+int input_mouse_scroll(void);
+
+/* Raw keyboard access for text-like input in editor */
+bool input_key_pressed(SDL_Scancode key);
+bool input_key_held(SDL_Scancode key);
+
 #endif /* JNR_INPUT_H */