Trim randomization elements to ChainLen and add verifyOrRevert

[parsaaba]

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[parsaaba]

Hey @rcarback
really like the WOTS+ work here.
noticed generateRandomizationElements derives all 67 elements, but chain only ever uses the first ChainLen (16), so 51 were wasted keccak calls. Trimming the loop saves ~6.5% gas on verify with identical behavior.
Also tucked in a few small things, verifyOrRevert so a false return can't be ignored, pinned the pragma to 0.8.34, made sign/generateKeyPair internal (offchain reference only), and added some negative/fuzz tests.
Happy to split this up or drop anything you'd rather not take. Thanks!