chore: scaffold tracks and tooling

Co-Authored-By: Claude Opus 5.5 <[email protected]>

fsharp/tests/Fixtures.fs

@@ -0,0 +1,21 @@
+/// Loads shared fixtures from ../../fixtures/<name>.json.
+/// Typed decoders (point, direction, status, action, state, opt) arrive with the model (Task 13).
+module Snake.Tests.Fixtures
+
+open System.IO
+open System.Text.Json
+
+let private dir =
+    Path.GetFullPath(Path.Combine(Directory.GetCurrentDirectory(), "..", "fixtures"))
+
+let load (name: string) : JsonElement =
+    use doc = JsonDocument.Parse(File.ReadAllText(Path.Combine(dir, name + ".json")))
+    doc.RootElement.Clone()
+
+/// The cases array of a fixture.
+let cases (name: string) : JsonElement list =
+    (load name).GetProperty("cases").EnumerateArray() |> List.ofSeq
+
+/// Decode a nullable JSON value into an Option.
+let opt (f: JsonElement -> 'a) (v: JsonElement) : 'a option =
+    if v.ValueKind = JsonValueKind.Null then None else Some(f v)