feat(fsharp): m11 pong capstone (model, rules, view, app) with tests

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

fsharp/solutions/Pong/Model.fs

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+/// m11 pong model: the same ideas as the snake model, for a different game.
+module Snake.Pong.Model
+
+open Snake.Model
+
+/// The ball's direction of travel, one cell per frame on each axis.
+type Velocity = { Dx: int; Dy: int }
+
+/// Which of the player's two keys are being held down right now.
+type Held = { Up: bool; Down: bool }
+
+type Score = { Player: int; Cpu: int }
+
+// RequireQualifiedAccess: write `PongStatus.Playing`, `Paddle.Up`, `PongAction.Frame`, so these cases never get
+// mixed up with the snake's `Playing`, `Up` and `Frame`.
+[<RequireQualifiedAccess>]
+type PongStatus =
+    | Playing
+    | Over
+
+/// The two keys that move the player's paddle.
+[<RequireQualifiedAccess>]
+type Paddle =
+    | Up
+    | Down
+
+/// A whole match. Positions are in cells: `Ball` is a cell of the 80x50 court; `Player` and `Cpu` are the y of the
+/// top cell of each paddle. `Seed` is the unsigned 32-bit random seed used to pick the direction of each serve.
+type PongState =
+    { Ball: Point
+      Vel: Velocity
+      Player: int
+      Cpu: int
+      Held: Held
+      Score: Score
+      Status: PongStatus
+      Seed: uint32
+      Frame: int }
+
+/// Everything that can happen in a match, as data.
+[<RequireQualifiedAccess>]
+type PongAction =
+    | Press of Paddle
+    | Release of Paddle
+    | Frame
+    | Restart
+
+// The court, in cells.
+let W = 80
+let H = 50
+
+/// Pixels per cell.
+let PCELL = 8
+
+/// Paddle height in cells.
+let PADDLE = 8
+
+/// Column of the player's paddle (left) and of the cpu's paddle (right).
+let PLAYER_X = 2
+let CPU_X = 77
+
+/// Points needed to win the match.
+let WIN = 5
+
+let initialPong (seed: uint32) : PongState =
+    { Ball = { X = 40; Y = 25 }
+      Vel = { Dx = 1; Dy = 1 }
+      Player = 21
+      Cpu = 21
+      Held = { Up = false; Down = false }
+      // `Score.Player` says which record is meant: PongState has Player and Cpu fields too.
+      Score = { Score.Player = 0; Cpu = 0 }
+      Status = PongStatus.Playing
+      Seed = seed
+      Frame = 0 }