Goal

The daemon reacts to seats being added or removed at runtime (hot-plugged GPU docks,
USB-attached seat devices, external monitors).

Scope

• udev DRM monitor — add a udev_monitor fd to the event loop; receive
  drm uevents for display connector changes (add / remove / change).
• Seat added/removed — subscribe to logind's SeatNew and SeatRemoved
  D-Bus signals; call seat_add() / seat_remove() and start or stop the
  greeter accordingly.
• CanGraphical gate — query logind's CanGraphical property on each seat
  before starting a greeter. Skip seats where the property is false (no monitor
  attached). Subscribe to PropertiesChanged to start/stop greeters when
  monitors are plugged or unplugged.
• Display detection — drm_seat_has_display() sysfs enumeration to detect
  connected connectors before starting a greeter on a non-seat0 seat. Check at
  both initial seat discovery and greeter restart — this is what prevents
  crash-loops on monitorless seats, and once it is in place the default
  restart-delay can be reduced from 2 s to ~0.5 s.

Removes the /* SHORTCUT */ from Phase 2 (no hotplug monitoring).

Design notes

The first Phase 11 attempt revealed several architectural issues that need
to be resolved up-front before writing action code:

• Two event sources (udev DRM + CanGraphical PropertiesChanged) overlap for
  the same real-world events, causing a double-SIGTERM hazard. Decide which
  source is authoritative for each decision before wiring up callbacks.
• sd_bus_slot * must be saved so PropertiesChanged subscriptions can be
  cancelled on seat removal.
• seat_remove() with a static array invalidates struct seat * pointers
  held in event userdata. Consider switching to a linked list first.

See tmp/dev-hotplug-issues.md for the full set of issues identified during
the first attempt (branch dev-hotplug).

Verification

Run atrium. Plug/unplug a USB seat device; confirm the event is detected
and the seat is started/stopped accordingly. Confirm monitorless seats are
skipped cleanly. Test hotplug with an active session on an adjacent seat.