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.c

@@ -85,6 +85,8 @@ bool tilemap_load(Tilemap *map, const char *path, SDL_Renderer *renderer) {
             map->player_spawn = vec2(sx * TILE_SIZE, sy * TILE_SIZE);
         } else if (strncmp(line, "GRAVITY ", 8) == 0) {
             sscanf(line + 8, "%f", &map->gravity);
+        } else if (strncmp(line, "WIND ", 5) == 0) {
+            sscanf(line + 5, "%f", &map->wind);
         } else if (strncmp(line, "TILEDEF ", 8) == 0) {
             int id, tx, ty;
             uint32_t flags;