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.

rules/rules_test.go

@@ -0,0 +1,82 @@
+package rules
+
+import "testing"
+
+func TestEnglishConsistency(t *testing.T) {
+	rs := English()
+	if err := rs.Validate(); err != nil {
+		t.Fatalf("Validate: %v", err)
+	}
+	if rs.Rows != 15 || rs.Cols != 15 {
+		t.Errorf("board = %dx%d, want 15x15", rs.Rows, rs.Cols)
+	}
+	if rs.Size() != 26 {
+		t.Errorf("alphabet size = %d, want 26", rs.Size())
+	}
+	if rs.Center != 7*15+7 {
+		t.Errorf("centre = %d, want %d", rs.Center, 7*15+7)
+	}
+
+	letters := 0
+	for _, c := range rs.Counts {
+		letters += c
+	}
+	if letters != 98 {
+		t.Errorf("sum(Counts) = %d, want 98", letters)
+	}
+	if rs.Blanks != 2 || letters+rs.Blanks != 100 {
+		t.Errorf("bag = %d letters + %d blanks, want 98+2=100", letters, rs.Blanks)
+	}
+
+	points := 0
+	for i := range rs.Values {
+		points += rs.Values[i] * rs.Counts[i]
+	}
+	if points != 187 {
+		t.Errorf("total bag points = %d, want 187", points)
+	}
+}
+
+func TestEnglishPremiums(t *testing.T) {
+	rs := English()
+
+	spot := []struct {
+		r, c int
+		want Premium
+	}{
+		{0, 0, TW}, {0, 7, TW}, {0, 14, TW}, {7, 0, TW}, {14, 7, TW},
+		{7, 7, DW}, {1, 1, DW}, {4, 4, DW},
+		{1, 5, TL}, {5, 5, TL}, {9, 9, TL},
+		{0, 3, DL}, {3, 0, DL}, {6, 6, DL},
+		{0, 1, None}, {7, 1, None},
+	}
+	for _, s := range spot {
+		if got := rs.Premium(s.r, s.c); got != s.want {
+			t.Errorf("Premium(%d,%d) = %d, want %d", s.r, s.c, got, s.want)
+		}
+	}
+
+	// Census of premium squares for the standard board.
+	census := map[Premium]int{}
+	for i := range rs.Rows * rs.Cols {
+		census[rs.PremiumAt(i)]++
+	}
+	want := map[Premium]int{None: 164, DL: 24, TL: 12, DW: 17, TW: 8}
+	for p, n := range want {
+		if census[p] != n {
+			t.Errorf("premium %d count = %d, want %d", p, census[p], n)
+		}
+	}
+
+	// The standard board is symmetric under transpose and 180° rotation.
+	for r := range rs.Rows {
+		for c := range rs.Cols {
+			if rs.Premium(r, c) != rs.Premium(c, r) {
+				t.Errorf("not transpose-symmetric at (%d,%d)", r, c)
+			}
+			if rs.Premium(r, c) != rs.Premium(rs.Rows-1-r, rs.Cols-1-c) {
+				t.Errorf("not 180°-symmetric at (%d,%d)", r, c)
+			}
+		}
+	}
+}