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/levelgen.h

@@ -46,6 +46,13 @@ typedef struct LevelGenConfig {
 /* Default config for quick generation */
 LevelGenConfig levelgen_default_config(void);
 
+/* Config preset for a space station level:
+ * very long horizontally, narrow playable area,
+ * near-zero gravity, all THEME_SPACE_STATION.
+ * depth (0-based) escalates difficulty and length
+ * with each subsequent station level. */
+LevelGenConfig levelgen_station_config(uint32_t seed, int depth);
+
 /* Generate a level directly into a Tilemap struct.
  * The Tilemap will have layers allocated, tile defs set,
  * entity spawns populated, and metadata configured.
@@ -53,6 +60,12 @@ LevelGenConfig levelgen_default_config(void);
  * Returns true on success. */
 bool levelgen_generate(Tilemap *map, const LevelGenConfig *config);
 
+/* Generate a space-station level: very long, narrow,
+ * near-zero gravity. Uses its own segment pool tuned
+ * for horizontal layouts in tight corridors.
+ * Returns true on success. */
+bool levelgen_generate_station(Tilemap *map, const LevelGenConfig *config);
+
 /* Dump a generated (or any) Tilemap to a .lvl file.
  * Useful for inspecting/editing procedural output.
  * Returns true on success. */