I haven't decided what to do with the timed close -- mainly we want to send a close but not hang indefinitely. The question is, what's the protocol beyond that? Abort?
Add sSkipAll.
ZGuardMtxR now only does a Acquire/Release on the transition/to from its own counter's zero. Which should be functionally equivalent unless non stack-based guards are in use.
Add sWriteFully and sThrowEndOfChanW.