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/drone.c

@@ -65,7 +65,7 @@ static void drone_update(Entity *self, float dt, const Tilemap *map) {
             self->body.pos.y + self->body.size.y * 0.5f
         );
         particle_emit_spark(center, (SDL_Color){50, 200, 255, 255});
-        self->flags |= ENTITY_DEAD;
+        entity_destroy(s_em, self);
         return;
     }