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/tilemap.h

@@ -50,6 +50,7 @@ typedef struct Tilemap {
     char         tileset_path[ASSET_PATH_MAX]; /* tileset file path */
     Vec2         player_spawn;
     float        gravity;           /* level gravity (px/s^2), 0 = use default */
+    float        wind;              /* horizontal wind force (px/s^2), +=right  */
     char         music_path[ASSET_PATH_MAX]; /* level music file path          */
     SDL_Color    bg_color;          /* background clear color                  */
     bool         has_bg_color;      /* true if BG_COLOR was set                */