chore: scaffold tracks and tooling

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

scala/test/Fixtures.test.scala

@@ -0,0 +1,18 @@
+package snake
+
+import java.nio.file.{Files, Paths}
+
+/** Loads shared fixtures from ../fixtures/<name>.json.
+  * Typed decoders (point, direction, status, action, state, opt) arrive with the model (Task 11). */
+object Fixtures:
+  private val dir = Paths.get(sys.props.getOrElse("user.dir", ".")).resolve("..").resolve("fixtures").normalize
+
+  def load(name: String): ujson.Value =
+    ujson.read(Files.readString(dir.resolve(s"$name.json")))
+
+  /** The cases array of a fixture. */
+  def cases(name: String): Seq[ujson.Value] = load(name)("cases").arr.toSeq
+
+  /** Decode a nullable JSON value into an Option. */
+  def opt[A](v: ujson.Value)(f: ujson.Value => A): Option[A] =
+    if v.isNull then None else Some(f(v))