A leaderboard is a claim: these people did this thing, in this order. If you cannot check the claim, you do not have a leaderboard — you have a form that accepts numbers.
The hard part is that our ten games have almost nothing in common mechanically, so there is no single way to verify them. We ended up with three.
Replay the moves
reconstruct and hue are move-based. The client sends the log of swaps, and the server replays them against the puzzle it generated. Either the moves sort the board or they do not, and either the count matches what was claimed or it does not. Nothing is trusted.
Grade the answer
thread, deduce, reveal and rebus each have a single correct answer. So the server never sends it. The puzzle your device receives has the solution stripped out, guesses go up to be graded, and the answer stays where you cannot read it.
Check every guess, then sign the win
warmer, groups, spot and arc give feedback continuously — a warmth rank, a “one away”, a hit or a miss. Each guess is evaluated server-side, and a completed run ends with a signed token that the score submission has to carry. You cannot mint the token without having played the guesses that lead to it.
The rule underneath all three
Hints are free and hinted runs are unranked. That one line resolves most of the tension between making a game welcoming and making its leaderboard mean something: help yourself as much as you like, just not on the board.