fix(engine): EvaluatePlay honors the single-word rule

The move preview (EvaluatePlay) validated under standard rules — it called
ValidatePlay without the game's play options — so under the single-word
rule it rejected a play whose only flaw was incidental invalid perpendicular
cross-words, even though SubmitPlay accepts it. The UI gates the submit
button on the preview, so such a play (e.g. КРАН bridging an existing Р on
the test contour) could not be made.

Pass g.playOpts() via ValidatePlayOpts, mirroring Play, so the preview's
legality and score match submission. Robots are unaffected — they search
via GenerateMovesOpts and submit via Play, both already opts-aware — and a
regression test asserts that too.

backend/internal/engine/domain.go

@@ -92,10 +92,12 @@ func (g *Game) SubmitExchange(tiles []string) (MoveRecord, error) {
 
 // EvaluatePlay scores and validates a tentative play without committing it,
 // backing the unlimited "what would my next move score, and is it legal?" tool.
-// It infers the play's orientation from the tiles and the board exactly as
-// SubmitPlay does, then returns the decoded move (placed tiles, the words it
-// forms, its orientation and its score) or ErrIllegalPlay when the solver
-// rejects it. The board, racks, bag and turn are left untouched.
+// It infers the play's orientation from the tiles and the board and applies the
+// game's play options exactly as SubmitPlay does, so under the single-word rule
+// perpendicular cross-words are ignored: the preview's legality and score then
+// match what submitting the play would yield. It returns the decoded move (placed
+// tiles, the words it forms, its orientation and its score) or ErrIllegalPlay when
+// the solver rejects it. The board, racks, bag and turn are left untouched.
 func (g *Game) EvaluatePlay(tiles []TileRecord) (MoveRecord, error) {
 	if g.over {
 		return MoveRecord{}, ErrGameOver
@@ -104,7 +106,7 @@ func (g *Game) EvaluatePlay(tiles []TileRecord) (MoveRecord, error) {
 	if err != nil {
 		return MoveRecord{}, err
 	}
-	move, err := g.solver.ValidatePlay(g.board, resolveDirection(g.board, placements), placements)
+	move, err := g.solver.ValidatePlayOpts(g.board, resolveDirection(g.board, placements), placements, g.playOpts())
 	if err != nil {
 		return MoveRecord{}, fmt.Errorf("%w: %v", ErrIllegalPlay, err)
 	}