Accept alternative checkmating moves in puzzles

[Qelxiros]

Summary

Puzzle validation previously accepted only the single scripted UCI move from the Lichess solution line. That's too strict in one well-defined case: when a player finds a different checkmate than the one scripted. A mate ends the game — the puzzle is solved regardless of which mating move was played. This matches how Lichess itself handles it.

Change

validateAndApply now accepts an off-solution move if pos.Update(mv).Status() == chess.Checkmate, and marks the puzzle solved immediately (even if the scripted line had more moves — e.g. the player found a shorter mate).

The relaxation is intentionally narrow — only mating alternatives are accepted:

• The puzzle data is a single PGN line with no eval annotations, so there's no reliable signal for "equally good non-mate move."
• Accepting non-forced alternatives would break multi-move puzzles, since the scripted engine reply only makes sense as a response to the scripted move.

ELO submission is driven by client-side solved/unsolved state (the server doesn't re-validate moves), so this client-side change is sufficient.

Tests

• TestPuzzleAcceptsAlternativeMate — multiple queen mates available; scripted line is Qh1#, player plays Qh2# → accepted, state Success.
• TestPuzzleRejectsNonMateAlternative — an off-solution non-mating move (Qa2) → still Failure.

All client tests pass.

🤖 Generated with Claude Code

[ikopke23]

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