lldpd: run on physicals and expose neighbour topology as the lldp fact (#513)

## Why

LLDP is the only source of physical switch/port topology in the estate: which switch and which port each machine is cabled to exists nowhere else (not in intent YAML, not in any other fact, not in DHCP/DNS). NetBox needs it to record device interface -> switch/port connections. This installs and enables lldpd on physical hosts and exposes its neighbour data as a structured `lldp` fact for the terraform-infra pdbmux backfill to consume (linked issue below).

## Changes

- Add `profiles::lldpd`: install the `lldpd` package and enable+start 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 receive it.
- Add the `lldp` custom fact (`modules/libs/lib/facter/lldp.rb`): parse `lldpctl -f json0` into a per-interface map of `{neighbor_chassis_name, neighbor_chassis_mac, neighbor_chassis_descr, neighbor_port_id, neighbor_port_descr, vlan_id, vlan_name}`, skipping interfaces with no neighbour. Confined to physical Linux hosts that have `lldpctl` and a live lldpd socket; returns an empty hash on any error so it can never break a puppet run.

json0 is used because it array-wraps every node regardless of cardinality, so one neighbour and many neighbours parse identically (plain `keyvalue` folds the neighbour SysName into the key path; plain `json` collapses single-element arrays into objects).

## Sample fact output (prodnxsr0019, enp1s0)

```json
{
  "enp1s0": {
    "neighbor_chassis_name":  "sg3429x-m2-02",
    "neighbor_chassis_mac":   "b8:fb:b3:a7:f9:5d",
    "neighbor_chassis_descr": "48-Port Gigabit Smart Managed Pro Switch",
    "neighbor_port_id":       "24",
    "neighbor_port_descr":    "two-gigabitEthernet 1/0/11",
    "vlan_id":                "201",
    "vlan_name":              "fabric_common_02"
  }
}
```

## Consumer

NetBox population of the switch/port cabling is tracked in terraform-infra: unkin/terraform-infra#7 (extend the pdbmux backfill / PR #6 to read this fact and emit the connection).

https://claude.ai/code/session_01JUoARVdmhxKQHyyyp1pxeT
Reviewed-on: #513
Co-authored-by: Ben Vincent <ben@unkin.net>
Co-committed-by: Ben Vincent <ben@unkin.net>

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')}": {}

modules/libs/lib/facter/lldp.rb

@@ -0,0 +1,104 @@
+# frozen_string_literal: true
+
+require 'facter'
+require 'json'
+
+# Exposes LLDP neighbour topology (switch/port each interface is cabled to) as
+# the structured `lldp` fact, keyed by local interface. This is the only source
+# of physical switch/port topology in the estate and feeds NetBox. Uses
+# `lldpctl -f json0`: json0 wraps every node in an array regardless of
+# cardinality, so one neighbour and many neighbours parse identically (plain
+# `keyvalue` folds the neighbour's sysname into the key path, and plain `json`
+# collapses single-element arrays into objects). Never raises: any error or a
+# down daemon yields an empty hash so a puppet run can never break.
+module LldpFact
+  SOCKETS = ['/run/lldpd.socket', '/var/run/lldpd.socket'].freeze
+
+  module_function
+
+  # First element of a json0 node (everything is array-wrapped), or the value
+  # itself if it is not an array; nil when absent.
+  def first(node)
+    node.is_a?(Array) ? node[0] : node
+  end
+
+  # Array form of a json0 node whatever its cardinality.
+  def list(node)
+    node.is_a?(Array) ? node : [node].compact
+  end
+
+  # Value string of a json0 leaf like [{ 'value' => 'x' }].
+  def leaf(node)
+    entry = first(node)
+    entry.is_a?(Hash) ? entry['value'] : entry
+  end
+
+  # Chassis MAC from its id list, preferring the entry typed 'mac'.
+  def chassis_mac(chassis)
+    ids = list(chassis['id'])
+    mac = ids.find { |id| id.is_a?(Hash) && id['type'] == 'mac' } || ids.first
+    mac.is_a?(Hash) ? mac['value'] : nil
+  end
+
+  # Topology record for one local interface, or nil when it has no neighbour.
+  def neighbour(iface)
+    chassis = first(iface['chassis'])
+    port    = first(iface['port'])
+    return nil unless chassis && port
+
+    chassis_fields(chassis).merge(port_fields(port, first(iface['vlan'])))
+  end
+
+  def chassis_fields(chassis)
+    {
+      'neighbor_chassis_name' => leaf(chassis['name']),
+      'neighbor_chassis_mac' => chassis_mac(chassis),
+      'neighbor_chassis_descr' => leaf(chassis['descr'])
+    }
+  end
+
+  def port_fields(port, vlan)
+    port_id = first(port['id'])
+    vlan_h  = vlan.is_a?(Hash) ? vlan : {}
+    {
+      'neighbor_port_id' => port_id.is_a?(Hash) ? port_id['value'] : port_id,
+      'neighbor_port_descr' => leaf(port['descr']),
+      'vlan_id' => vlan_h['vlan-id'],
+      'vlan_name' => vlan_h['value']
+    }
+  end
+
+  def interfaces(output)
+    lldp = first(JSON.parse(output)['lldp']) || {}
+    list(lldp['interface'])
+  end
+
+  # Map of local interface => topology record, skipping interfaces with no
+  # neighbour.
+  def collect(ifaces)
+    ifaces.each_with_object({}) do |iface, acc|
+      next unless iface.is_a?(Hash)
+
+      name = iface['name']
+      data = neighbour(iface)
+      acc[name] = data if name && data
+    end
+  end
+
+  def resolve
+    output = Facter::Core::Execution.execute('lldpctl -f json0 2>/dev/null', on_fail: nil)
+    return {} if output.to_s.empty?
+
+    collect(interfaces(output))
+  rescue StandardError
+    {}
+  end
+end
+
+Facter.add(:lldp) do
+  confine kernel: 'Linux'
+  confine { Facter.value(:is_virtual) == false }
+  confine { Facter::Core::Execution.which('lldpctl') }
+  confine { LldpFact::SOCKETS.any? { |path| File.exist?(path) } }
+  setcode { LldpFact.resolve }
+end

site/profiles/manifests/lldpd.pp

@@ -0,0 +1,33 @@
+# profiles::lldpd
+#
+# Runs lldpd on physical hosts so each machine learns its switch/port topology
+# via LLDP. The `lldp` fact exposes that neighbour data for NetBox. Assigned
+# via hiera_include from hieradata/virtual/physical.yaml (physicals only); the
+# lldpd.service ships disabled, so it is explicitly enabled and started here.
+class profiles::lldpd (
+  Boolean $enabled = true,
+  String  $package = 'lldpd',
+  String  $service = 'lldpd',
+){
+
+  if $enabled {
+    package { $package:
+      ensure => installed,
+    }
+
+    service { $service:
+      ensure    => running,
+      enable    => true,
+      subscribe => Package[$package],
+    }
+  } else {
+    service { $service:
+      ensure => stopped,
+      enable => false,
+    }
+
+    package { $package:
+      ensure => absent,
+    }
+  }
+}