The compute node's room for interactive sessions (notebooks), for the
caller: the effective node limit (the node's ceiling, or the quota
table's LIMITS#ALL when lower), how many are open by anyone, how many
are the caller's, and the caller's resolved per-user limit (user
override, then LIMITS#DEFAULT). The run-creation gate decides on the
same limits, so a client can grey out a full node before a user
starts a notebook there.

Advisory, not a reservation: a notebook opening or closing between
this call and a submission can change the answer.