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/Model.fs

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+/// m02 model: the data the whole game is built from. Records and unions are immutable.
+module Snake.Model
+
+/// A grid cell. Records compare structurally: two points with the same X and Y are `=`.
+type Point = { X: int; Y: int }
+
+type Direction =
+    | Up
+    | Down
+    | Left
+    | Right
+
+type Status =
+    | Playing
+    | Paused
+    | GameOver
+    | Won
+
+/// Everything that can happen to the game, as data.
+type Action =
+    | Tick
+    | Turn of Direction
+    | TogglePause
+    | Restart
+    | Frame
+
+/// The whole game. `Snake` is head first; `Seed` is the unsigned 32-bit random seed.
+type GameState =
+    { Width: int
+      Height: int
+      Snake: Point list
+      Dir: Direction
+      NextDir: Direction
+      Apple: Point option
+      Seed: uint32
+      Score: int
+      Status: Status
+      Frame: int }
+
+let delta (dir: Direction) : Point =
+    match dir with
+    | Up -> { X = 0; Y = -1 }
+    | Down -> { X = 0; Y = 1 }
+    | Left -> { X = -1; Y = 0 }
+    | Right -> { X = 1; Y = 0 }
+
+let opposite (dir: Direction) : Direction =
+    match dir with
+    | Up -> Down
+    | Down -> Up
+    | Left -> Right
+    | Right -> Left
+
+let initialState (seed: uint32) : GameState =
+    { Width = 20
+      Height = 20
+      Snake = [ { X = 10; Y = 10 }; { X = 9; Y = 10 }; { X = 8; Y = 10 } ]
+      Dir = Right
+      NextDir = Right
+      Apple = None
+      Seed = seed
+      Score = 0
+      Status = Playing
+      Frame = 0 }