migrate restart to opt-in UFCS (sx a47ea14)

Free function restart(board, seed) is dot-called from main.sx and
board_view.sx; the sx opt-in UFCS change gates plain functions out of
dot-dispatch, so declare it ufcs. ios-sim build green, 23/23 logic
tests.

board.sx

@@ -900,7 +900,7 @@ reshuffle_if_deadlocked :: (board: *Board) -> bool {
 // seed → identical starting layout), zeroes `score` and `moves_made`, and
 // restores the default move budget and score goal, so `level_status` reads
 // `in_progress` again. The entry point P7.2's restart button calls.
-restart :: (board: *Board, seed: i64) {
+restart :: ufcs (board: *Board, seed: i64) {
     board.init(seed);
 }