chore: scaffold tracks and tooling

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

scala/test/Fixtures.test.scala

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+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))

scala/test/M00Sanity.test.scala

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+package snake
+
+class M00Sanity extends munit.FunSuite:
+  test("arithmetic") { assertEquals(1 + 1, 2) }
+
+  test("fixture loads and cases hold") {
+    val fx = Fixtures.load("m00-sanity")
+    assertEquals(fx("function").str, "sanity")
+    Fixtures.cases("m00-sanity").foreach { c =>
+      val a = c("args").arr
+      assertEquals(a(0).num.toInt + a(1).num.toInt, c("expected").num.toInt, c("name").str)
+    }
+  }
+
+  test("opt decodes null") {
+    assertEquals(Fixtures.opt(ujson.Null)(_.num), None)
+    assertEquals(Fixtures.opt(ujson.Num(3))(_.num.toInt), Some(3))
+  }