feat(fsharp): m01-m06 util, model, rules, random, spawn, game, view with Expecto tests

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

fsharp/solutions/Rules.fs

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+/// m03 rules: the pure game rules. Each function takes a state and returns a new one.
+module Snake.Rules
+
+open Snake.Model
+
+let move (p: Point) (dir: Direction) : Point =
+    let d = delta dir
+    { X = p.X + d.X; Y = p.Y + d.Y }
+
+let inBounds (state: GameState) (p: Point) : bool =
+    p.X >= 0 && p.X < state.Width && p.Y >= 0 && p.Y < state.Height
+
+// Validate against `Dir` (last moved), not `NextDir`: Up then Left within one tick must not fold the snake back on itself.
+let turn (state: GameState) (dir: Direction) : GameState =
+    if state.Status = Playing && dir <> opposite state.Dir then { state with NextDir = dir } else state
+
+let advance (state: GameState) : GameState =
+    if state.Status <> Playing then
+        state
+    else
+        let d = state.NextDir
+        let head = move (List.head state.Snake) d
+        let eating = state.Apple = Some head
+        // When not eating, the tail leaves its cell this step, so the head may move into it.
+        let body = if eating then state.Snake else Util.dropLast state.Snake
+
+        if not (inBounds state head) || Util.contains body head (=) then
+            { state with Dir = d; Status = GameOver }
+        else
+            { state with
+                Snake = head :: body
+                Dir = d
+                Apple = if eating then None else state.Apple
+                Score = state.Score + (if eating then 1 else 0) }