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.

docs/ARCHITECTURE.md

@@ -285,8 +285,9 @@ Key points:
 - **Multiple words per turn (Russian games).** Russian variants carry a per-game
   **single-word rule**, chosen on New Game (default **off** = single word; on = standard
   Scrabble). Off, only the **main word** along the play direction is validated and scored —
-  perpendicular cross-words are ignored, including in robot move generation; on, every
-  cross-word must be a real word and is scored. The engine threads it as
+  perpendicular cross-words are ignored, including in robot move generation and the
+  unlimited move preview; on, every cross-word must be a real word and is scored. The
+  engine threads it as
   `scrabble.PlayOptions{IgnoreCrossWords}` (solver `v1.1.0`); connectivity and the
   first-move centre rule are unaffected. The "Russian-only" limit is a **UI affordance**:
   the backend and engine are variant-agnostic about the flag, and English games always send