feat(nx-rt-core): take the next message the system has posted

[LNSD]

Second part of the applet message pump to move. Acting on a message was already here; getting one was not, so the pump had a body and no way to be fed.

• poll_message waits on the message event with a zero timeout and reads the queue when it is signalled, which is the poll a program drives its own loop from
• Nothing waiting is Ok(None) rather than an error: it is what most passes of a loop find, and a caller should not have to sort it out of a failure
• A queue that will not hand over a waiting message is reported rather than fatal, unlike the C path that aborts the process on it: nothing is consumed by a refused request, so the caller loses only this pass
• The wait deliberately leaves the event signalled, as the system does: clearing it here would open a window in which an arriving message signals nothing and is noticed only when the next one follows it
• No override changed and no C symbol moved; the pump that joins these two parts comes next