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/engine/parallax.h

@@ -43,6 +43,7 @@ typedef enum ParallaxStyle {
     PARALLAX_STYLE_ALIEN_SKY,   /* alien planet surface: dusty, hazy       */
     PARALLAX_STYLE_INTERIOR,    /* indoor base: panels, pipes, structural  */
     PARALLAX_STYLE_DEEP_SPACE,  /* space station windows: vivid stars      */
+    PARALLAX_STYLE_MOON,        /* moon surface: craters, grey terrain     */
 } ParallaxStyle;
 
 /* Generate both layers with a unified style */