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/projectile.c

@@ -204,6 +204,12 @@ static void projectile_update(Entity *self, float dt, const Tilemap *map) {
         if (body->vel.y > MAX_FALL_SPEED) body->vel.y = MAX_FALL_SPEED;
     }
 
+    /* ── Apply wind ─────────────────────────── */
+    float wind = physics_get_wind();
+    if (wind != 0.0f) {
+        body->vel.x += wind * dt;
+    }
+
     /* ── Move ────────────────────────────────── */
     body->pos.x += body->vel.x * dt;
     body->pos.y += body->vel.y * dt;