Retry budget tokens

Compute how many retries a client-side retry budget permits. Maintain a token balance starting at min_budget. Process an ordered list of events: a 'req' event adds 'ratio' tokens to the balance (ratio is the retries-per-request allowance, e.g. 0.2); a 'retry' event is allowed only if the balance is >= 1.0, in which case it consumes 1.0 token and counts as allowed, otherwise it is denied (balance unchanged). Return the total number of retries that were allowed. ratio is a non-negative float; min_budget is a non-negative int.