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

assets/levels/level01.lvl

@@ -16,6 +16,15 @@ ENTITY grunt 9  16
 ENTITY flyer 20 10
 ENTITY flyer 30 8
 
+# Powerups
+ENTITY powerup_hp 13 12
+ENTITY powerup_jet 21 14
+ENTITY powerup_drone 29 16
+
+# Exit zone: EXIT <tile_x> <tile_y> <tile_w> <tile_h> [target_path]
+# Inside the walled room at the far right
+EXIT 35 16 2 3 assets/levels/level02.lvl
+
 # Hazards
 ENTITY turret 33 4
 ENTITY flame_vent 18 19