Record B-53's fixing commit in BUGS.md

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

BUGS.md

@@ -11,7 +11,7 @@ next to the fix so `git log --all --grep 'B-nn'` finds it later.
 
 Already fixed and removed: B-52 (a round past ~50 participants deadlocked the
 payout — `025754c`), B-53 (a bet could pay into the pool of a round it was left
-out of — `PLACEHOLDER`).
+out of — `64f6229`).
 
 State of the tree at audit time: 264 unit tests, all passing; `tests/integration/`
 still empty; withdrawal and the RBF bump path still never live-broadcast.