An end-of-print auto-off on a plug that powers a filter fan marked the linked
printer offline and forced its state to "unknown". The mark was unrecoverable:
connected heals on the next MQTT message but state does not (only frames
carrying gcode_state rewrite it, and steady-state push_status frames are
partial), so the printer stayed "unknown" until a manual Force Refresh and the
queue never dispatched to it again.
The offline mark is now an explicit presumption: mark_power_off records the
state it overwrites and _on_message undoes it as soon as the printer sends
another report on its own topic, since inbound traffic proves the power was
never cut. A reconnect discards the saved state, so a genuine power cut is
unaffected. Each plug also gains a controls_printer_power flag (default true,
backfilled) that gates all five power-off paths, and the queue's power-on step
now picks the flagged plug instead of whichever linked plug came first.