Release LGMP stream slots before dispatching clipboard data so an X11
INCR consumer cannot prevent control queue acknowledgements.
Retain one bounded record in client memory while the sink is blocked.
Cancel abandoned X11 transfers after requestor destruction or a finite
progress timeout.
Mark the last non-empty FILE_DATA payload with BEGIN and END when the
producer knows it is terminal. Complete producer bookkeeping in the
following fileDataEnd call without consuming another LGMP grant. Keep
the standalone terminal record for empty and legacy unknown streams.
Drain up to 64 queued clipboard records per worker pass instead of
sleeping after every message. Poll at 1 ms while transfers, blocked
writes, held input, or pending output are active, and retain the 10 ms
interval while idle.
This removes a serialized grant round trip from every non-empty file
response and reduces scheduling delay while a large transfer is active.
Keep the 1 MiB payload size because the bottleneck was stop-and-wait
latency, not the chunk capacity.
Exercise a full 1 MiB response in a single BEGIN|END grant and retain
coverage for empty unknown-size responses.
