Fix asteroid double-impact and levelgen theme fallback

Explode asteroid on player contact instead of letting it continue
falling to the ground for a second impact. Return a safe default
theme when theme_count is zero to avoid reading uninitialized data.

src/game/hazards.c

@@ -699,10 +699,17 @@ static void asteroid_update(Entity *self, float dt, const Tilemap *map) {
                 player->body.pos.y + player->body.size.y * 0.5f
             );
             particle_emit_spark(hit_pos, (SDL_Color){180, 140, 80, 255});
+            particle_emit_death_puff(hit_pos, (SDL_Color){140, 110, 80, 255});
 
             if (s_asteroid_sfx_loaded) {
                 audio_play_sound(s_sfx_asteroid_impact, 80);
             }
+
+            /* Explode on contact — hide and start respawn */
+            ad->falling = false;
+            ad->respawn_timer = ASTEROID_RESPAWN;
+            self->body.pos.y = -100.0f;
+            return;
         }
     }
 

src/game/levelgen.c

@@ -602,7 +602,7 @@ static void gen_transition(Tilemap *map, int x0, int w,
 
 /* Get the theme for a given segment index from the config */
 static LevelTheme theme_for_segment(const LevelGenConfig *config, int seg_idx) {
-    if (config->theme_count <= 0) return config->themes[0];
+    if (config->theme_count <= 0) return THEME_PLANET_SURFACE;
     if (seg_idx >= config->theme_count) return config->themes[config->theme_count - 1];
     return config->themes[seg_idx];
 }