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.

backend/internal/engine/domain.go

@@ -62,7 +62,7 @@ func (g *Game) SubmitPlay(tiles []TileRecord) (MoveRecord, error) {
 	if err != nil {
 		return MoveRecord{}, err
 	}
-	return g.Play(resolveDirection(g.board, placements), placements)
+	return g.Play(g.playDirection(placements), placements)
 }
 
 // SubmitPlayDir is SubmitPlay with the orientation supplied rather than inferred.
@@ -106,7 +106,7 @@ func (g *Game) EvaluatePlay(tiles []TileRecord) (MoveRecord, error) {
 	if err != nil {
 		return MoveRecord{}, err
 	}
-	move, err := g.solver.ValidatePlayOpts(g.board, resolveDirection(g.board, placements), placements, g.playOpts())
+	move, err := g.solver.ValidatePlayOpts(g.board, g.playDirection(placements), placements, g.playOpts())
 	if err != nil {
 		return MoveRecord{}, fmt.Errorf("%w: %v", ErrIllegalPlay, err)
 	}
@@ -169,6 +169,33 @@ func (g *Game) placements(tiles []TileRecord) ([]scrabble.Placement, error) {
 	return out, nil
 }
 
+// playDirection resolves the orientation for a live play. resolveDirection infers it from
+// geometry alone, preferring the longer word when a single tile abuts the board on both
+// axes; under the single-word rule that can pick an orientation whose word is not in the
+// dictionary while the other orientation's is. So for a single tile under that rule the
+// engine tries both orientations through the solver and keeps the higher-scoring legal one
+// (horizontal breaks a tie). Multi-tile plays, and every play under the standard rule, keep
+// the geometric resolution: a multi-tile play's orientation is fixed by the line its tiles
+// share, and under the standard rule every word the play forms must be valid regardless of
+// which one is named the main word.
+func (g *Game) playDirection(placements []scrabble.Placement) scrabble.Direction {
+	geo := resolveDirection(g.board, placements)
+	if len(placements) != 1 || g.multipleWords {
+		return geo
+	}
+	best, found, bestScore := geo, false, 0
+	for _, dir := range [...]scrabble.Direction{scrabble.Horizontal, scrabble.Vertical} {
+		m, err := g.solver.ValidatePlayOpts(g.board, dir, placements, g.playOpts())
+		if err != nil {
+			continue
+		}
+		if !found || m.Score > bestScore {
+			best, found, bestScore = dir, true, m.Score
+		}
+	}
+	return best
+}
+
 // encodeTiles encodes decoded exchange tiles ("?" for a blank, otherwise a
 // concrete letter) into the internal byte form, wrapping a bad letter as
 // ErrTilesNotOnRack (the caller cannot hold a tile it cannot name).