feat(fsharp): m07 io and m08 stream with tests

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

fsharp/src/Stream.fs

@@ -0,0 +1,49 @@
+/// m08 stream: an event stream as a pure automaton (a Mealy machine).
+/// A step feeds one event and returns the (possibly empty) list of outputs it produced and the stream to use for
+/// the next event. Nothing is mutated, so a stream value can be run any number of times with the same results.
+module Snake.Stream
+
+/// Something that happens to the program: time passing (ms since start) or a key going down or up.
+type Event =
+    | Time of int64
+    | KeyDown of string
+    | KeyUp of string
+
+type Stream<'a> = Stream of (Event -> 'a list * Stream<'a>)
+
+/// Feed one event to `s`: the outputs it produced and the stream for the next event.
+let step (e: Event) (s: Stream<'a>) : 'a list * Stream<'a> = failwith "TODO m08"
+
+/// Emits the `key` of every KeyDown event; ignores everything else.
+/// (The stub is wrapped in `Stream` so the TODO only fires when the stream is stepped.)
+let keyDowns: Stream<string> = Stream(fun _ -> failwith "TODO m08")
+
+/// Emits the `key` of every KeyUp event; ignores everything else.
+let keyUps: Stream<string> = Stream(fun _ -> failwith "TODO m08")
+
+/// On `Time t`, emits every multiple `k*ms` with `last < k*ms <= t`, ascending, where `last` starts at 0
+/// and becomes `t` afterwards. A big time jump can emit several values. Ignores key events.
+/// Hint: write a private helper that carries `last`.
+let interval (ms: int64) : Stream<int64> = failwith "TODO m08"
+
+/// Apply `f` to every output.
+let map (f: 'a -> 'b) (s: Stream<'a>) : Stream<'b> = failwith "TODO m08"
+
+/// Keep only the outputs for which `p` is true.
+let filter (p: 'a -> bool) (s: Stream<'a>) : Stream<'a> = failwith "TODO m08"
+
+/// Apply `f` to every output; `Some b` emits `b`, `None` is dropped (like `List.choose`).
+let choose (f: 'a -> 'b option) (s: Stream<'a>) : Stream<'b> = failwith "TODO m08"
+
+/// Both streams step on every event; the outputs of `a` come before the outputs of `b`.
+let merge (a: Stream<'a>) (b: Stream<'a>) : Stream<'a> = failwith "TODO m08"
+
+/// Fold the outputs into an accumulator, starting from `z`; emits each new accumulator (not `z` itself).
+let scan (f: 'b -> 'a -> 'b) (z: 'b) (s: Stream<'a>) : Stream<'b> = failwith "TODO m08"
+
+/// Emit `v` once, before the outputs of the first step; later steps are unaffected.
+/// Note: streams only act when an event arrives, so if no event ever arrives, `v` is never emitted.
+let startWith (v: 'a) (s: Stream<'a>) : Stream<'a> = failwith "TODO m08"
+
+/// Feed all events through the stream and collect every output, in order. A pure fold.
+let run (events: Event list) (s: Stream<'a>) : 'a list = failwith "TODO m08"