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/level02.lvl

@@ -0,0 +1,59 @@
+# Level 02 - Space Station
+# ========================
+
+TILESET assets/tiles/tileset.png
+SIZE 30 23
+SPAWN 3 18
+GRAVITY 600
+BG_COLOR 10 10 25
+MUSIC assets/sounds/algardalgar.ogg
+
+# Enemies
+ENTITY grunt 12 18
+ENTITY flyer 18 10
+ENTITY grunt 22 14
+
+# Powerups
+ENTITY powerup_hp 10 11
+
+# Exit zone at the far right — transitions to procedural space station
+EXIT 27 16 2 3 generate:station
+
+# Hazards
+ENTITY turret 25 5
+ENTITY flame_vent 14 19
+ENTITY platform 8 13
+ENTITY platform_v 20 10
+
+# Tile definitions: ID TEX_X TEX_Y FLAGS
+# Flag 1 = SOLID, Flag 2 = PLATFORM
+TILEDEF 1 0 0 1
+TILEDEF 2 1 0 1
+TILEDEF 3 2 0 1
+TILEDEF 4 0 1 2
+
+# Collision layer (30 wide x 23 tall)
+LAYER collision
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