fix(input): start games with the plus key

Match the recovered Win16 registration flow: the first plus press creates Player 1 and additional presses add Players 2 through 4 until the initial launch. Down now belongs only to charging and releasing the plunger.

Test Plan:
- cargo fmt --check
- cargo test
- cargo clippy --all-targets --all-features -- -D warnings
- live: verify Down cannot leave attract mode
- live: start with plus, add Player 2, hold Down through partial/full plunger frames, and release onto the table
- git diff --check

tdkpin-rs/src/game.rs

@@ -92,6 +92,12 @@ pub struct Ball {
     pub in_launcher: bool,
 }
 
+#[derive(Clone, Copy, Debug, PartialEq, Eq)]
+enum PlayerEntry {
+    Open,
+    Closed,
+}
+
 impl Default for Ball {
     fn default() -> Self {
         Self {
@@ -128,6 +134,7 @@ pub struct Game {
     nudge_meter: f32,
     stalled_for: f32,
     launcher_was_down: bool,
+    player_entry: PlayerEntry,
 }
 
 impl Game {
@@ -157,6 +164,7 @@ impl Game {
             nudge_meter: 0.0,
             stalled_for: 0.0,
             launcher_was_down: false,
+            player_entry: PlayerEntry::Open,
         }
     }
 
@@ -168,12 +176,21 @@ impl Game {
         LAUNCHER_FRAME_THRESHOLDS.partition_point(|threshold| self.launcher_charge >= *threshold)
     }
 
+    pub fn add_player_before_launch(&mut self) -> bool {
+        if self.player_entry == PlayerEntry::Closed || self.players.len() >= 4 {
+            return false;
+        }
+        self.players.push(Player::default());
+        true
+    }
+
     fn fire_launcher(&mut self) {
         let launch_speed = LAUNCH_SPEED_MIN + self.launcher_charge * LAUNCH_SPEED_RANGE;
         self.ball.in_launcher = false;
         self.ball.velocity = vec2(0.0, -launch_speed);
         self.launcher_charge = 0.0;
         self.launcher_was_down = false;
+        self.player_entry = PlayerEntry::Closed;
         self.stalled_for = 0.0;
     }
 
@@ -626,6 +643,22 @@ mod tests {
         assert_eq!(Game::new(99).players.len(), 4);
     }
 
+    #[test]
+    fn plus_adds_players_only_before_the_first_launch() {
+        let mut game = Game::new(1);
+
+        assert!(game.add_player_before_launch());
+        assert!(game.add_player_before_launch());
+        assert!(game.add_player_before_launch());
+        assert!(!game.add_player_before_launch());
+        assert_eq!(game.players.len(), 4);
+
+        let mut game = Game::new(1);
+        launch_ball(&mut game, 1);
+        assert!(!game.add_player_before_launch());
+        assert_eq!(game.players.len(), 1);
+    }
+
     #[test]
     fn ball_starts_in_the_recovered_shooter_lane() {
         let game = Game::new(1);