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/editor.h

@@ -0,0 +1,98 @@
+#ifndef JNR_EDITOR_H
+#define JNR_EDITOR_H
+
+#include "engine/tilemap.h"
+#include "engine/camera.h"
+#include <SDL2/SDL.h>
+#include <stdbool.h>
+
+/* ═══════════════════════════════════════════════════
+ *  Level Editor
+ *
+ *  An in-engine level editor that runs as an
+ *  alternative game mode. Uses the same renderer,
+ *  camera, and tilemap systems as gameplay.
+ *
+ *  Tile palette is auto-discovered from the tileset
+ *  texture. Entity palette is auto-discovered from
+ *  the entity registry. Adding new tiles or entities
+ *  to the game makes them appear in the editor
+ *  automatically.
+ * ═══════════════════════════════════════════════════ */
+
+typedef enum EditorTool {
+    TOOL_PENCIL,        /* paint tiles                    */
+    TOOL_ERASER,        /* clear tiles (paint tile 0)     */
+    TOOL_FILL,          /* flood fill area                */
+    TOOL_ENTITY,        /* place/select entities          */
+    TOOL_SPAWN,         /* set player spawn point         */
+    TOOL_COUNT
+} EditorTool;
+
+typedef enum EditorLayer {
+    EDITOR_LAYER_COLLISION,
+    EDITOR_LAYER_BG,
+    EDITOR_LAYER_FG,
+    EDITOR_LAYER_COUNT
+} EditorLayer;
+
+/* UI panel regions */
+#define EDITOR_TOOLBAR_H   14   /* top bar height                  */
+#define EDITOR_PALETTE_W   80   /* right panel width               */
+#define EDITOR_STATUS_H    10   /* bottom status bar height        */
+
+typedef struct Editor {
+    /* ── Map data ──────────────────────────── */
+    Tilemap       map;
+    char          file_path[256];
+    bool          has_file;         /* true if loaded from / saved to file */
+    bool          dirty;            /* unsaved changes                     */
+
+    /* ── Camera ────────────────────────────── */
+    Camera        camera;
+
+    /* ── Tool state ────────────────────────── */
+    EditorTool    tool;
+    EditorLayer   active_layer;
+    uint16_t      selected_tile;    /* currently selected tile ID          */
+    int           selected_entity;  /* index into entity registry          */
+    bool          show_grid;
+    bool          show_all_layers;  /* dim inactive layers                 */
+
+    /* ── Entity editing ────────────────────── */
+    int           dragging_entity;  /* index in entity_spawns, or -1       */
+    float         drag_offset_x;   /* offset from entity origin to mouse  */
+    float         drag_offset_y;
+
+    /* ── Palette scroll ────────────────────── */
+    int           tile_palette_scroll;
+    int           entity_palette_scroll;
+
+    /* ── Tileset info (auto-discovered) ────── */
+    int           tileset_cols;
+    int           tileset_rows;
+    int           tileset_total;    /* total tiles in tileset              */
+
+    /* ── UI state ──────────────────────────── */
+    bool          active;           /* editor is running                   */
+} Editor;
+
+/* Lifecycle */
+void editor_init(Editor *ed);
+void editor_new_level(Editor *ed, int width, int height);
+bool editor_load(Editor *ed, const char *path);
+bool editor_save(Editor *ed);
+bool editor_save_as(Editor *ed, const char *path);
+void editor_free(Editor *ed);
+
+/* Frame callbacks (plug into engine) */
+void editor_update(Editor *ed, float dt);
+void editor_render(Editor *ed, float interpolation);
+
+/* Returns true if the user pressed the test-play key */
+bool editor_wants_test_play(Editor *ed);
+
+/* Returns true if the editor wants to quit */
+bool editor_wants_quit(Editor *ed);
+
+#endif /* JNR_EDITOR_H */