Add optional macvlan network type alongside existing NAT mode. Changes include:
- Config schema: three new fields (network_type, macvlan_parent, macvlan_gateway)
- All 7 example incus.yaml files updated with commented-out macvlan fields
- Network creation logic branched on incus_project_network_type
- Profile NIC configuration branched for macvlan (nictype=macvlan parent=)
- Config validation script checks required macvlan fields when type=macvlan
- SPEC.md and README.md updated with macvlan documentation