This ticket adds a stopgap backpressure signal that complements the main in-flight bytes and batch count signal (SERVER-134780): the dispatcher blocks when any single worker's queued bytes exceeds a per-worker cap (e.g., 100 MB).

This ticket should include:

• Byte accounting at enqueue/consume — each WorkItem carries its serialized byte size, and the pool tracks per-worker queued bytes.
• CV wait — the dispatcher blocks on a condition variable after flushing a batch's slices if any worker is over its byte threshold, woken by workers as they drain (the design here can be improved by the implementer).
• A threshold server parameter (IDL) for per-worker queued bytes.