Survive a rate below one per second

Here is the bug that ships.

Plenty of real limits are slower than one request per second — one upload every four seconds, thirty API calls a minute. Run your bucket at refillPerSecond = 0.5 and watch what happens if the refill rounds down: each call adds floor(1 × 0.5) = 0 tokens and advances the bookmark, so the remaining half-token is thrown away every single time. The bucket refills never, and the client is throttled forever.

capacity 2, 0.5 tokens/second
t=0   allow -> true   allow -> true    // empty
t=1   allow -> false                   // half a token is not a token
t=2   allow -> true                    // the halves added up

The fix is to stop throwing the remainder away: hold the balance as a fraction and only insist on a whole token at the moment one is spent. tokens() already floors for display, which is the right place for rounding — at the boundary, not in the accounting.