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/testdata/singleword/cross_invalid.gcg

@@ -0,0 +1,11 @@
+#character-encoding UTF-8
+#player1 p1 Player One
+#player2 p2 Player Two
+#note Single-word rule fixture ("multiple words per turn" off). After the opening, each AT
+#note stacks under the previous one, forming an invalid vertical cross-word (TT, then AAA /
+#note TTT, ...). Standard rules reject these; the single-word rule accepts them and scores
+#note the main word only. The +SCORE/CUMUL columns are main-word-only scores.
+>p1: ATEIOUN 8H AT +4 4
+>p2: ATRSEOD 9H AT +3 3
+>p1: ATLNGUP 10H AT +2 6
+>p2: ATCHIMB 11H AT +2 5