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/engine/physics.c

@@ -4,9 +4,11 @@
 #include <math.h>
 
 static float s_gravity = DEFAULT_GRAVITY;
+static float s_wind    = 0.0f;
 
 void physics_init(void) {
     s_gravity = DEFAULT_GRAVITY;
+    s_wind    = 0.0f;
 }
 
 void physics_set_gravity(float gravity) {
@@ -17,6 +19,14 @@ float physics_get_gravity(void) {
     return s_gravity;
 }
 
+void physics_set_wind(float wind) {
+    s_wind = wind;
+}
+
+float physics_get_wind(void) {
+    return s_wind;
+}
+
 static void resolve_tilemap_x(Body *body, const Tilemap *map) {
     if (!map) return;
 
@@ -102,6 +112,12 @@ void physics_update(Body *body, float dt, const Tilemap *map) {
     /* Apply gravity */
     body->vel.y += s_gravity * body->gravity_scale * dt;
 
+    /* Apply wind — halved on ground (friction counteracts) */
+    if (s_wind != 0.0f && body->gravity_scale > 0.0f) {
+        float wind_factor = body->on_ground ? 0.5f : 1.0f;
+        body->vel.x += s_wind * body->gravity_scale * wind_factor * dt;
+    }
+
     /* Clamp fall speed */
     if (body->vel.y > MAX_FALL_SPEED) {
         body->vel.y = MAX_FALL_SPEED;