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/entity_registry.h

@@ -0,0 +1,52 @@
+#ifndef JNR_ENTITY_REGISTRY_H
+#define JNR_ENTITY_REGISTRY_H
+
+#include "engine/entity.h"
+#include <SDL2/SDL.h>
+
+/* ═══════════════════════════════════════════════════
+ *  Entity Registry
+ *
+ *  Central table that maps entity type name strings
+ *  to their spawn functions and metadata. Used by:
+ *  - Level loader (to spawn entities from .lvl files)
+ *  - Level editor (to discover available entity types)
+ *
+ *  When adding a new entity type to the game, add one
+ *  entry here and it will appear in the editor palette
+ *  automatically.
+ * ═══════════════════════════════════════════════════ */
+
+#define MAX_REGISTRY_ENTRIES 32
+
+/* Spawn function: creates an entity at position, returns it */
+typedef Entity *(*EntitySpawnFn)(EntityManager *em, Vec2 pos);
+
+typedef struct EntityRegEntry {
+    const char   *name;         /* .lvl file name, e.g. "grunt"          */
+    const char   *display;      /* human-readable, e.g. "Grunt"          */
+    EntitySpawnFn spawn_fn;     /* function that creates the entity      */
+    SDL_Color     color;        /* editor palette color for preview      */
+    int           width;        /* hitbox width  (for editor preview)    */
+    int           height;       /* hitbox height (for editor preview)    */
+} EntityRegEntry;
+
+typedef struct EntityRegistry {
+    EntityRegEntry entries[MAX_REGISTRY_ENTRIES];
+    int            count;
+} EntityRegistry;
+
+/* Global registry instance */
+extern EntityRegistry g_entity_registry;
+
+/* Call once at startup to populate the registry.
+ * Also registers entity behaviors with the given EntityManager. */
+void entity_registry_init(EntityManager *em);
+
+/* Look up a registry entry by name (returns NULL if not found) */
+const EntityRegEntry *entity_registry_find(const char *name);
+
+/* Spawn an entity by registry name. Returns NULL if name unknown. */
+Entity *entity_registry_spawn(EntityManager *em, const char *name, Vec2 pos);
+
+#endif /* JNR_ENTITY_REGISTRY_H */