Implement Scrabble move generator (DAWG) with English and Russian rules

A Go library that returns every legal play ranked by score and scores or
validates plays, using the Appel-Jacobson DAWG algorithm over
github.com/iliadenisov/dafsa v1.1.0.

- DAWG move generation (across / down / both), full tournament scoring with a
  per-tile breakdown; public Solver: GenerateMoves (ranked), ScorePlay,
  ValidatePlay.
- Rulesets: English Scrabble, Russian Scrabble, Эрудит (parameterizable Ruleset).
- cmd/builddict (build the DAWG from the dictionaries submodule), cmd/stress
  (self-play benchmark), selfplay engine; brute-force test oracle.
- A GADDAG was implemented, benchmarked and removed (the DAWG was smaller and
  faster for a scoring solver); see RESULTS.md and ALGORITHM.md.

scrabble/key.go

@@ -0,0 +1,36 @@
+package scrabble
+
+import (
+	"sort"
+	"strconv"
+	"strings"
+)
+
+// moveKey is a canonical string identifying a play (direction plus its placed tiles),
+// used to de-duplicate and compare generated moves.
+func moveKey(dir Direction, p []Placement) string {
+	ps := append([]Placement(nil), p...)
+	sort.Slice(ps, func(i, j int) bool {
+		if ps[i].Row != ps[j].Row {
+			return ps[i].Row < ps[j].Row
+		}
+		return ps[i].Col < ps[j].Col
+	})
+	var sb strings.Builder
+	sb.WriteByte('0' + byte(dir))
+	for _, pl := range ps {
+		sb.WriteByte(';')
+		sb.WriteString(strconv.Itoa(pl.Row))
+		sb.WriteByte(',')
+		sb.WriteString(strconv.Itoa(pl.Col))
+		sb.WriteByte(',')
+		sb.WriteString(strconv.Itoa(int(pl.Letter)))
+		if pl.Blank {
+			sb.WriteByte('*')
+		}
+	}
+	return sb.String()
+}
+
+// Key returns the canonical identifier of the move (direction plus its placed tiles).
+func (m Move) Key() string { return moveKey(m.Dir, m.Tiles) }