fix(input): require plus to start players

Remove the Enter shortcut so the start flow matches the original plus-key handler. Use the recovered player-add sound resource for both the first and subsequent players.

Test Plan:
- cargo fmt --check
- cargo test
- cargo clippy --all-targets --all-features -- -D warnings
- git diff --cached --check

tdkpin-rs/README.md

@@ -51,7 +51,7 @@ and `claw-18`; `--seed N` fixes random choices for later seeded scenarios.
 
 | Action | Original key | Additional modern key |
 | --- | --- | --- |
-| Start game / add up to 4 players | `+` | `=` or Enter starts one player |
+| Start game / add up to 4 players | `+` | `=` uses the same main-keyboard key |
 | Charge launcher | Hold Down arrow, release to launch | - |
 | Left flipper | Left Ctrl | `A` or Left arrow |
 | Right flipper | Keypad Enter | Right Ctrl, `D`, or Right arrow |