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/game/powerup.h

@@ -0,0 +1,40 @@
+#ifndef JNR_POWERUP_H
+#define JNR_POWERUP_H
+
+#include "engine/entity.h"
+
+/* ═══════════════════════════════════════════════════
+ *  Powerup Pickups
+ *
+ *  Small collectible items that grant the player
+ *  an instant benefit on contact:
+ *  - Health:   restores 1 HP
+ *  - Jetpack:  instantly refills all dash charges
+ *  - Drone:    spawns an orbiting combat drone
+ * ═══════════════════════════════════════════════════ */
+
+typedef enum PowerupKind {
+    POWERUP_HEALTH,
+    POWERUP_JETPACK,
+    POWERUP_DRONE,
+    POWERUP_KIND_COUNT
+} PowerupKind;
+
+typedef struct PowerupData {
+    PowerupKind kind;
+    float       bob_timer;    /* sine bob animation */
+    Vec2        base_pos;     /* original spawn position */
+} PowerupData;
+
+/* Register the powerup entity type */
+void powerup_register(EntityManager *em);
+
+/* Spawn a specific powerup at a position */
+Entity *powerup_spawn(EntityManager *em, Vec2 pos, PowerupKind kind);
+
+/* Convenience spawners for level/levelgen */
+Entity *powerup_spawn_health(EntityManager *em, Vec2 pos);
+Entity *powerup_spawn_jetpack(EntityManager *em, Vec2 pos);
+Entity *powerup_spawn_drone(EntityManager *em, Vec2 pos);
+
+#endif /* JNR_POWERUP_H */