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/powerup.h

@@ -17,6 +17,7 @@ typedef enum PowerupKind {
     POWERUP_HEALTH,
     POWERUP_JETPACK,
     POWERUP_DRONE,
+    POWERUP_GUN,
     POWERUP_KIND_COUNT
 } PowerupKind;
 
@@ -36,5 +37,6 @@ Entity *powerup_spawn(EntityManager *em, Vec2 pos, PowerupKind kind);
 Entity *powerup_spawn_health(EntityManager *em, Vec2 pos);
 Entity *powerup_spawn_jetpack(EntityManager *em, Vec2 pos);
 Entity *powerup_spawn_drone(EntityManager *em, Vec2 pos);
+Entity *powerup_spawn_gun(EntityManager *em, Vec2 pos);
 
 #endif /* JNR_POWERUP_H */