Add per-level wind atmosphere property

WIND directive in .lvl files sets a constant horizontal force (px/s^2)
that pushes entities, projectiles, and particles. Positive is rightward.

Wind is applied as acceleration in physics_update() (halved on ground),
directly to projectile and particle velocities, and as a gentle position
drift on flyers. Entities with gravity_scale=0 (drones, spacecraft) are
unaffected. Levels default to no wind when the directive is absent.

src/game/editor.c

@@ -450,6 +450,8 @@ static bool save_tilemap(const Tilemap *map, const char *path) {
 
     if (map->gravity > 0)
         fprintf(f, "GRAVITY %.0f\n", map->gravity);
+    if (map->wind != 0.0f)
+        fprintf(f, "WIND %.0f\n", map->wind);
     if (map->has_bg_color)
         fprintf(f, "BG_COLOR %d %d %d\n", map->bg_color.r, map->bg_color.g, map->bg_color.b);
     if (map->music_path[0])