scrabble: single-word rule requires in-line connection

Under PlayOptions.IgnoreCrossWords a play connected to the board through any
perpendicular contact, even one forming a non-word, because connected() reused
the standard rule's touchesPerpendicular test while cross-words were suppressed.
That let an all-new word merely abut the board sideways and be accepted, and the
generator, with relaxed cross-sets, offered such plays.

Require the main word to run through an existing tile along the play direction
when cross-words are ignored, and filter generated moves by the same predicate so
generation agrees with ValidatePlayOpts. Standard rules are unchanged.

scrabble/move.go

@@ -79,7 +79,8 @@ 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.
+	// direction must be a valid word. That main word must still connect by running through
+	// an existing tile along its line — a tile that only abuts the board perpendicular to
+	// that line does not connect — and the first-move centre rule still applies.
 	IgnoreCrossWords bool
 }