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

Makefile

@@ -13,6 +13,8 @@ ifdef WASM
              -sSDL2_IMAGE_FORMATS='["png"]' \
              -sSDL2_MIXER_FORMATS='["ogg"]' \
              -sALLOW_MEMORY_GROWTH=1 \
+             -sEXPORTED_FUNCTIONS='["_main","_editor_upload_flag_ptr","_editor_save_flag_ptr","_editor_load_flag_ptr","_editor_load_vfs_file","_malloc","_free"]' \
+             -sEXPORTED_RUNTIME_METHODS='["UTF8ToString","stringToUTF8","lengthBytesUTF8"]' \
              --preload-file assets \
              --shell-file web/shell.html