Replace the builtin EthernetSwitch stub with a Linux kernel bridge backed
by uBridge's brctl module. Each switch node creates one kernel bridge
(gns3br{N}) with VLAN filtering; each port is a persistent TAP enslaved to
the bridge, relayed by a per-port uBridge bridge (nio_tap <-> nio_udp).
- Access/dot1q/qinq port modes translated to brctl vlan primitives
- Compute router repointed from Dynamips to Builtin manager
- Tests updated: 21 router-level tests + 215 surrounding tests pass
- Real-kernel e2e verified: access 100 PVID untagged, dot1q trunk
VIDs 1-4094 + native 1, qinq 802.1ad proto + 200 PVID
Ethertype 0x9100/0x9200 handling and port-count guard relaxation are
deferred pending resolution.
