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/levelgen.c

@@ -1631,6 +1631,10 @@ bool levelgen_dump_lvl(const Tilemap *map, const char *path) {
         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);