fix: checkout helper logic

[DaniilSyzenko]

Changes checkout helper logic.

• Removes impossible checkout suggestions like "D22" (it doesn't exist on the board)
• Improves suggestion logic to prioritize more practical combinations, e.g., "D15, T19" instead of "T10, T19" because triples are
• Removes unnecessary checks for triple bull, as isValidSector already returns null for rawValue > 20

[BySplashGm]

Thanks for fixing the invalid sectors (D21, D22, D24 don't exist — those were real bugs).

The score_validator.dart cleanup is fine assuming isValidSector already guards against rawValue > 20, which it does.

Main concern: the new checkout suggestions for 2- and 3-dart finishes heavily favour starting on a Double (e.g. 62: ['D11', 'D20'], 66: ['D13', 'D20']). In practice, triples are easier to aim for than doubles — the segments are larger — so standard checkout charts always prioritise Triples as the first dart. For example:

• 62 → T10, D16 or T14, D10
• 66 → T10, D18
• 81 → T19, D12

The old code was closer to convention here, even if it had some invalid sectors. The fix should address the invalid sectors without changing the Triple-first logic.

Do you have unit tests covering CheckoutHelper? Given how many values changed, tests would help catch any arithmetic errors.

[DaniilSyzenko]

Hi, can I still work on this?