scrabble: add single-word-per-turn rule via PlayOptions

Add an optional "single word per turn" rule: only the main word along the
play direction is validated and scored; perpendicular cross-words are ignored
— not formed, validated, scored, or used to constrain move generation. The
zero PlayOptions stays standard Scrabble, so existing callers are unchanged.

- PlayOptions{IgnoreCrossWords} threaded through new EvaluateOpts,
  Solver.{ScorePlay,ValidatePlay,GenerateMoves}Opts and the DAWG generator
  (relaxed cross-sets via fullSet, main-word-only scoring).
- connected() tests perpendicular adjacency directly instead of via
  Move.Cross, so an all-new main word touching the board only sideways still
  connects when cross-words are suppressed (behaviour-preserving for standard
  play).
- Tests: focused corner-case suite (solver_opts_test.go) and a single-word
  GCG fixture; the 17 real-game GCG fixtures stay green as the standard-rules
  regression guard.

scrabble/score.go

@@ -40,8 +40,17 @@ func perpendicular(d Direction) Direction {
 // dir under ruleset rs. It validates geometry — the tiles lie on one line, on empty
 // squares, and form a single contiguous run together with existing tiles — but does not
 // check the dictionary or board connectivity; ValidatePlay layers those on top. tiles
-// need not be sorted.
+// need not be sorted. It evaluates under standard rules; use EvaluateOpts to apply optional
+// rule variations.
 func Evaluate(b *board.Board, rs *rules.Ruleset, dir Direction, tiles []Placement) (Move, error) {
+	return EvaluateOpts(b, rs, dir, tiles, PlayOptions{})
+}
+
+// EvaluateOpts is Evaluate with optional rule variations (PlayOptions). With
+// opts.IgnoreCrossWords set, the play's perpendicular cross-words are neither formed nor
+// scored: the returned move carries no Cross words and its Score counts only the main word
+// (plus any all-tiles bonus).
+func EvaluateOpts(b *board.Board, rs *rules.Ruleset, dir Direction, tiles []Placement, opts PlayOptions) (Move, error) {
 	if len(tiles) == 0 {
 		return Move{}, errors.New("scrabble: empty play")
 	}
@@ -78,10 +87,12 @@ func Evaluate(b *board.Board, rs *rules.Ruleset, dir Direction, tiles []Placemen
 	}
 
 	move := Move{Dir: dir, Tiles: ts, Main: main, Score: main.Score}
-	for _, t := range ts {
-		if cw, ok := crossWord(b, rs, dir, t); ok {
-			move.Cross = append(move.Cross, cw)
-			move.Score += cw.Score
+	if !opts.IgnoreCrossWords {
+		for _, t := range ts {
+			if cw, ok := crossWord(b, rs, dir, t); ok {
+				move.Cross = append(move.Cross, cw)
+				move.Score += cw.Score
+			}
 		}
 	}
 	if len(ts) == rs.RackSize {