A session request refused with 400 or 409, or a +H channel message
refused for membership, a ban or +m, used to spend its stamp, so a
retry with the same stamp was refused as already used.
hashcash.Validator.Validate is split into Check, which spends
nothing, and Claim, which records the stamp; Release undoes a claim.
Session creation checks the stamp first, claims it just before
CreateSession and releases it if CreateSession fails.
SendChannelMessage checks the stamp after the membership, ban and +m
checks.
Model: opus-5-5
sneak