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/level.c

@@ -57,6 +57,9 @@ static bool level_setup(Level *level) {
         physics_set_gravity(DEFAULT_GRAVITY);
     }
 
+    /* Apply level wind (0 = no wind) */
+    physics_set_wind(level->map.wind);
+
     /* Apply level background color */
     if (level->map.has_bg_color) {
         renderer_set_clear_color(level->map.bg_color);