feat(fsharp): m09 input/app/runtime and m10 extensions with tests

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

fsharp/src/Input.fs

@@ -0,0 +1,10 @@
+/// m09 input: from keyboard keys to game actions.
+module Snake.Input
+
+open Snake.Model
+
+/// Translate a keyboard key name (as in the browser's `KeyboardEvent.key`) into the action it triggers, if any.
+/// m09: "ArrowUp"/"w"/"W" -> `Some (Turn Up)`, "ArrowDown"/"s"/"S" -> Down, "ArrowLeft"/"a"/"A" -> Left,
+/// "ArrowRight"/"d"/"D" -> Right. Any other key -> `None`.
+/// (m10) " " (Space) -> `Some TogglePause`, "r"/"R" -> `Some Restart`.
+let keyToAction (key: string) : Action option = failwith "TODO m09"