Function spawn_warming_creditor 

pub fn spawn_warming_creditor(
    allowance: Arc<WarmingAllowance>,
    metrics: Arc<Metrics>,
) -> (Arc<dyn WarmingCreditSink>, StopHandle)

Spawn the single background task that owns the credit queue and applies each credit to allowance, returning the WarmingCreditSink the serve path enqueues through and the StopHandle that stops the task.

This is what takes the bucket lock off the stream task (see the module Seam): a serve completion resolves the source from the tag map — one shard read, never the bucket lock — and does one bounded try_send. This task takes the bucket lock instead, where waiting behind the buy loop costs a queued credit rather than a held lane slot.

The handle is returned rather than detached so shutdown can await the drain and a task that died is visible as a join error instead of as credits that quietly stop landing. StopHandle::shutdown ends the loop after it flushes whatever is already queued (the handle owns the stop token beside the task, so the join cannot hang on the sinks the client handler holds); the loop also ends if every sink is dropped first.