Default node position for spine/core/border-leaf/leaf/access is now a
Y-tier by role (parsed from hostname, same trust level as the site
parsing from #49) with X grouped by site within each tier, replacing
the flat site-grid heuristic.
Position specifically is now read from the live Grafana dashboard before
computing anything, so a node manually drag-and-dropped in the UI keeps
its position across regeneration -- only nodes with no live position yet
(new devices, or first-ever run) get the layered default. Devices removed
from the topology are simply absent from this run's output, so no
orphaned position/link reference is possible.
Refs #53
Damien Arnodo
fcd7ea2567