fix(engine): single-word rule connects along the play line

Bumps the engine to scrabble-solver v1.1.1, where a single-word-per-turn play
must form its word along its own line through an existing tile: a multi-tile play
that touches the board only perpendicular to itself (the contour РЮМ/КЕД/ОР cases)
no longer connects. For a single tile that abuts the board on both axes the engine
now plays the higher-scoring legal orientation instead of the geometrically longer
one (playDirection), so a real word is never rejected in favour of a non-word.

Reworks the single-word solver/engine tests for the corrected rule (no longer a
superset of standard play) and updates ARCHITECTURE/FUNCTIONAL/PRERELEASE.

docs/ARCHITECTURE.md

@@ -301,10 +301,15 @@ Key points:
   **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 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**:
+  unlimited move preview; on, every cross-word must be a real word and is scored. The main
+  word must still run **through an existing tile along its own line** to connect: a play that
+  forms no word along the direction it is laid — touching the board only perpendicular to
+  itself — is illegal even though its cross-word is never checked, and for a single tile that
+  abuts the board on both axes the engine plays the higher-scoring legal orientation. The
+  single-word rule is therefore **not a superset** of the standard rule: it forbids parallel
+  plays the standard rule allows and admits in-line plays whose cross-words are invalid. The
+  engine threads it as `scrabble.PlayOptions{IgnoreCrossWords}` (solver `v1.1.1`); the
+  first-move centre rule is unaffected. The "Russian-only" limit is a **UI affordance**:
   the backend and engine are variant-agnostic about the flag, and English games always send
   it on (standard). For auto-match the rule is part of the matchmaking key, so only players
   who chose the same rule are paired (the rule field rides every create/enqueue request, so