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/move.go

@@ -72,3 +72,14 @@ type Move struct {
 	Bonus int         // all-tiles (bingo) bonus included in Score, or 0
 	Score int         // total: Main.Score + Σ Cross.Score + Bonus
 }
+
+// PlayOptions tunes optional rule variations applied when evaluating, validating or
+// generating plays. The zero value is standard Scrabble.
+type PlayOptions struct {
+	// IgnoreCrossWords makes a play's perpendicular cross-words irrelevant: they are not
+	// formed, validated, scored or used to constrain move generation. It expresses the
+	// "single word per turn" house rule, under which only the main word along the play
+	// direction must be a valid word; board connectivity and the first-move centre rule
+	// still apply.
+	IgnoreCrossWords bool
+}