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.

internal/graph/graph.go

@@ -0,0 +1,21 @@
+// Package graph provides thin, reusable helpers over a dafsa Cursor that the move
+// generator builds on. It keeps the rest of the solver from depending on dafsa
+// traversal details directly.
+package graph
+
+import dawg "github.com/iliadenisov/dafsa"
+
+// Spell follows the given alphabet indices from the cursor's root. It returns the
+// state reached, whether that state is accepting, and whether the whole path exists.
+// When ok is false the path ran into a missing edge; n and final are meaningless.
+func Spell(c *dawg.Cursor, indices []byte) (n dawg.Node, final, ok bool) {
+	n = c.Root()
+	final = c.Final(n)
+	for _, ix := range indices {
+		n, final, ok = c.Next(n, ix)
+		if !ok {
+			return n, false, false
+		}
+	}
+	return n, final, true
+}