fix: add human_friendly_id to 4 schemas for idempotent Repository sync

Add local_identifier attribute (kind: Text) + uniqueness_constraints +
human_friendly_id to InfraVlanVniMapping, InfraEVPNInstance,
InfraMlagInterface and InfraUnderlayLink so Infrahub can perform
upserts during Repository backend sync instead of duplicating objects
on every import.

Schema changes (following the InfraBGPPeerGroup/BGPSession pattern):
- infrahub/schemas/vlan_vxlan.yml: VlanVniMapping + EVPNInstance
- infrahub/schemas/mlag.yml: MlagInterface
- infrahub/schemas/extensions.yml: UnderlayLink

Object file updates:
- 06-vlans-vxlan.yml: local_identifier added to all VlanVniMapping and
  EVPNInstance entries (format: leaf1__vlan40__vni110040 / leaf1__vlan40)
- 12-mlag.yml: InfraMlagInterface section added for all 4 MLAG pairs
  (format: mlag1__leaf1-leaf2)
- 14-fabric-links.yml: InfraUnderlayLink section added for all 16
  spine↔leaf P2P links (format: spine1-eth1__leaf1-eth11)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

infrahub/schemas/vlan_vxlan.yml

@@ -174,8 +174,15 @@ nodes:
     label: VLAN-VNI Mapping
     icon: mdi--swap-horizontal
     include_in_menu: false
+    uniqueness_constraints:
+      - ["local_identifier__value"]
+    human_friendly_id:
+      - local_identifier__value
     display_label: "{{ description__value }}"
     attributes:
+      - name: local_identifier
+        kind: Text
+        description: "Unique identifier combining device name, VLAN ID and VNI (e.g. leaf1__vlan40__vni110040)"
       - name: description
         kind: Text
         optional: true
@@ -202,8 +209,15 @@ nodes:
     label: EVPN Instance
     icon: mdi--cloud-sync
     include_in_menu: false
+    uniqueness_constraints:
+      - ["local_identifier__value"]
+    human_friendly_id:
+      - local_identifier__value
     display_label: "{{ route_distinguisher__value }}"
     attributes:
+      - name: local_identifier
+        kind: Text
+        description: "Unique identifier combining device name and VLAN ID (e.g. leaf1__vlan40)"
       - name: route_distinguisher
         kind: Text
         description: "Route Distinguisher (format: ASN:VNI or IP:VNI)"