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.

selfplay/selfplay_test.go

@@ -0,0 +1,33 @@
+package selfplay_test
+
+import (
+	"testing"
+
+	"github.com/iliadenisov/alphabet"
+
+	"scrabble-solver/internal/dictdawg"
+	"scrabble-solver/internal/wordlist"
+	"scrabble-solver/rules"
+	"scrabble-solver/scrabble"
+	"scrabble-solver/selfplay"
+)
+
+func TestPlayGameSmoke(t *testing.T) {
+	rs := rules.English()
+	words := wordlist.Encode([]string{
+		"cat", "cats", "car", "care", "cares", "cot", "cap", "cab", "at", "as",
+		"tea", "eat", "ear", "era", "are", "oat", "oats", "sat", "set", "sea",
+		"tar", "tars", "star", "arts", "rat", "rats", "ace", "aces", "scar", "scare",
+	}, alphabet.Latin(), 2, 15)
+	f, err := dictdawg.Build(alphabet.Latin(), words)
+	if err != nil {
+		t.Fatal(err)
+	}
+	gen := scrabble.NewDAWGGenerator(rs, f)
+
+	res := selfplay.PlayGame(rs, gen, scrabble.Both, 1, nil)
+	if res.Turns == 0 || res.Plays == 0 {
+		t.Errorf("degenerate game: %+v", res)
+	}
+	t.Logf("smoke game: turns=%d plays=%d scores=%v", res.Turns, res.Plays, res.Scores)
+}