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/levelgen.h

@@ -0,0 +1,61 @@
+#ifndef JNR_LEVELGEN_H
+#define JNR_LEVELGEN_H
+
+#include "engine/tilemap.h"
+#include <stdint.h>
+
+/* ═══════════════════════════════════════════════════
+ *  Procedural Level Generator
+ *
+ *  Generates a Tilemap directly in memory by stitching
+ *  together segment templates. Supports:
+ *  - Mixed layouts (horizontal, vertical, arena)
+ *  - Enemy and hazard placement
+ *  - Difficulty scaling via seed
+ *  - Dumping to .lvl format for inspection
+ * ═══════════════════════════════════════════════════ */
+
+/* Level theme — affects layout, hazards, gravity, atmosphere */
+typedef enum LevelTheme {
+    THEME_PLANET_SURFACE,  /* alien planet exterior: open terrain, pits,
+                              flame vents, harsh conditions, high gravity */
+    THEME_PLANET_BASE,     /* research outpost on a planet: corridors,
+                              force fields, turrets, medium gravity       */
+    THEME_SPACE_STATION,   /* orbital station: shafts, platforms, low
+                              gravity, moving platforms, turrets          */
+    THEME_COUNT
+} LevelTheme;
+
+#define MAX_THEME_SEQUENCE 10
+
+/* Generator configuration */
+typedef struct LevelGenConfig {
+    uint32_t   seed;           /* RNG seed (0 = random from time)       */
+    int        num_segments;   /* how many segments to stitch (2-10)    */
+    float      difficulty;     /* 0.0 = easy, 1.0 = hard               */
+    float      gravity;        /* level gravity override (0 = auto)     */
+
+    /* Theme sequence: one theme per segment.
+     * If theme_count == 0, all segments use themes[0].
+     * If theme_count > 0, segment i uses themes[min(i, theme_count-1)].
+     * Transition segments are auto-inserted at theme boundaries. */
+    LevelTheme themes[MAX_THEME_SEQUENCE];
+    int        theme_count;    /* number of entries in themes[] */
+} LevelGenConfig;
+
+/* Default config for quick generation */
+LevelGenConfig levelgen_default_config(void);
+
+/* Generate a level directly into a Tilemap struct.
+ * The Tilemap will have layers allocated, tile defs set,
+ * entity spawns populated, and metadata configured.
+ * Caller must eventually call tilemap_free() on it.
+ * Returns true on success. */
+bool levelgen_generate(Tilemap *map, const LevelGenConfig *config);
+
+/* Dump a generated (or any) Tilemap to a .lvl file.
+ * Useful for inspecting/editing procedural output.
+ * Returns true on success. */
+bool levelgen_dump_lvl(const Tilemap *map, const char *path);
+
+#endif /* JNR_LEVELGEN_H */