Effect Token Bucket Shaper

Rate limiting that allows a real burst but bounds the true rate. A fixed window counted in wall-clock minutes admits 1000 at 0:59 and 1000 more at 1:00, so 2000 requests land in two seconds, the exact spike the limit forbids. A token bucket meters continuously: tokens refill at a steady rate with no boundary reset, and capacity sets the burst a normal page load is allowed before throughput settles to the refill rate. Lazy refill computes accrued tokens from elapsed logical time in one Ref.modify per request, so concurrent requests can never both spend the same token. The demo: 50 racing requests for a 1-token bucket admit exactly 1. Pinned to effect 4.0.0-beta.98.