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

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

fsharp/solutions/Runtime.fs

@@ -0,0 +1,46 @@
+/// m09 runtime: the single impure place in the course (with the web/ glue); everything else only builds values.
+/// Here we interpret IO descriptions against a Platform and hold the one mutable variable, the current stream.
+module Snake.Runtime
+
+open Snake.View
+open Snake.IO
+open Snake.Stream
+
+/// The outside world, as the runtime sees it.
+type Platform =
+    { FillRect: int -> int -> int -> int -> string -> unit
+      FillText: int -> int -> string -> string -> int -> string -> unit
+      RandomSeed: unit -> uint32
+      StartClock: (int64 -> unit) -> unit
+      OnKeyDown: (string -> unit) -> unit
+      OnKeyUp: (string -> unit) -> unit }
+
+// Perform one op on the platform and return its answer.
+let private perform (p: Platform) (op: Op) : OpResult =
+    match op with
+    | RandomSeed -> Seed(p.RandomSeed())
+    | Draw(Rect(x, y, w, h, color)) ->
+        p.FillRect x y w h color
+        Done
+    | Draw(Text(x, y, text, color, size, align)) ->
+        p.FillText x y text color size align
+        Done
+
+// Tail recursive, so it compiles to a loop: a scene is hundreds of ops and the stack must not grow with them.
+let rec runIO (p: Platform) (io: IO<'a>) : 'a =
+    match io with
+    | Pure a -> a
+    | Effect(op, k) -> runIO p (k (perform p op))
+
+let runApp (p: Platform) (main: IO<Stream<IO<unit>>>) : unit =
+    // The one mutable variable. F# closures cannot capture a `let mutable`, so it is a ref cell.
+    let stream = ref (runIO p main)
+
+    let feed (event: Event) =
+        let ios, next = Stream.step event stream.Value
+        stream.Value <- next
+        List.iter (runIO p) ios
+
+    p.OnKeyDown(fun key -> feed (KeyDown key))
+    p.OnKeyUp(fun key -> feed (KeyUp key))
+    p.StartClock(fun t -> feed (Time t))