lldpd: run on physicals and expose neighbour topology as the lldp fact

LLDP is the only source of physical switch/port topology in the estate; NetBox
has no other way to learn which switch and port each machine is cabled to. Run
lldpd on physical hosts and surface its neighbour data so the terraform-infra
pdbmux backfill can populate NetBox interface connections.

- add profiles::lldpd installing the lldpd package and enabling+starting the
  service (it ships disabled), the service subscribing to the package
- assign it physicals-only via hiera_include in hieradata/virtual/physical.yaml
  (facts.virtual == 'physical'), merged unique with the common hiera_include; VMs
  never get it
- add the `lldp` custom fact (modules/libs/lib/facter/lldp.rb) parsing
  `lldpctl -f json0` into a per-interface map of neighbour chassis/port/vlan,
  skipping interfaces with no neighbour; confined to physical Linux hosts with
  lldpctl and a live lldpd socket, and never raising (empty hash on any error)

Claude-Session: https://claude.ai/code/session_01JUoARVdmhxKQHyyyp1pxeT

hieradata/virtual/physical.yaml

@@ -1,3 +1,8 @@
 ---
+# physical hosts only (facts.virtual == 'physical'); merged 'unique' with the
+# common hiera_include in profiles::base.
+hiera_include:
+  - profiles::lldpd
+
 profiles::packages::include:
   "%{hiera('lm-sensors::package')}": {}