Spend a hashcash stamp only when the request succeeds (closes #158)

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

README.md

@@ -1404,7 +1404,8 @@ Include the hashcash stamp in the `meta` field:
 **Server validation:** The server checks that the stamp is well-formed, meets
 the required difficulty, is bound to the correct channel and message body, has a
 recent date, and has not been previously used. Spent stamps are cached for 1
-year to prevent replay attacks.
+year to prevent replay attacks. The stamp is checked after membership, bans and
+`+m`, so a message refused for one of those does not spend it.
 
 **Error responses:** If the channel requires hashcash and the stamp is missing,
 invalid, or replayed, the server returns `ERR_CANNOTSENDTOCHAN (404)` with a
@@ -3084,6 +3085,10 @@ limits that punish shared networks.
     - Stamp has not been used before (replay prevention; the spent stamps are
       kept in memory, so a restart forgets them)
 
+    The stamp is spent only when the session is created. A request refused with
+    400 or 409 leaves it unspent, and the client can send the same stamp again
+    with another nick or username.
+
 ### Stamp Format
 
 Standard hashcash format: