Add moon surface intro level with asteroid hazards and unarmed mechanics

Introduce moon01.lvl as the starting level — a pure jump-and-run intro
with no gun and no enemies, just platforming over gaps and dodging falling
asteroids. The player picks up their gun upon transitioning to level01.

New features:
- Moon tileset and PARALLAX_STYLE_MOON with crater terrain backgrounds
- Asteroid entity (ENT_ASTEROID): falls from sky, damages on contact,
  explodes on ground with particles, respawns after delay
- PLAYER_UNARMED directive disables gun for the level
- Pit rescue mechanic: falling costs 1 HP and auto-dashes upward
- Gun powerup entity type for future armed-pickup levels
- Segment-based procedural level generator with themed rooms
- Extended editor with entity palette and improved tile cycling
- Web shell improvements for Emscripten builds

src/game/level.h

@@ -14,6 +14,10 @@ typedef struct Level {
     Parallax      parallax;
     Music         music;
     bool          music_started;
+
+    /* ── Exit / transition state ─────────── */
+    bool          exit_triggered;              /* player entered an exit zone  */
+    char          exit_target[ASSET_PATH_MAX]; /* target level path            */
 } Level;
 
 bool level_load(Level *level, const char *path);
@@ -22,4 +26,8 @@ void level_update(Level *level, float dt);
 void level_render(Level *level, float interpolation);
 void level_free(Level *level);
 
+/* Returns true if the player has triggered a level exit.
+ * The target path is stored in level->exit_target. */
+bool level_exit_triggered(const Level *level);
+
 #endif /* JNR_LEVEL_H */