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/entity_registry.c

@@ -31,6 +31,10 @@ static Entity *spawn_powerup_drone(EntityManager *em, Vec2 pos) {
     return powerup_spawn_drone(em, pos);
 }
 
+static Entity *spawn_powerup_gun(EntityManager *em, Vec2 pos) {
+    return powerup_spawn_gun(em, pos);
+}
+
 /* ── Registry population ─────────────────────────── */
 
 static void reg_add(const char *name, const char *display,
@@ -62,6 +66,7 @@ void entity_registry_init(EntityManager *em) {
     force_field_register(em);
     powerup_register(em);
     drone_register(em);
+    asteroid_register(em);
 
     /* ════════════════════════════════════════════
      *  REGISTRY TABLE
@@ -85,6 +90,8 @@ void entity_registry_init(EntityManager *em) {
     reg_add("powerup_hp",    "Health Pickup",  spawn_powerup_health, (SDL_Color){255, 80, 80, 255},   12, 12);
     reg_add("powerup_jet",   "Jetpack Refill", spawn_powerup_jetpack,(SDL_Color){255, 200, 50, 255},  12, 12);
     reg_add("powerup_drone", "Drone Pickup",   spawn_powerup_drone,  (SDL_Color){80, 200, 255, 255},  12, 12);
+    reg_add("powerup_gun",   "Gun Pickup",    spawn_powerup_gun,    (SDL_Color){200, 200, 220, 255}, 12, 12);
+    reg_add("asteroid",      "Asteroid",      asteroid_spawn,       (SDL_Color){140, 110, 80, 255},  ASTEROID_WIDTH, ASTEROID_HEIGHT);
 
     printf("Entity registry: %d types registered\n", g_entity_registry.count);
 }