feat(esw): wire peer anchors straight into the switch bridge

The Ethernet switch's kernel fast path: instead of a link crossing the
switch's per-port uBridge relay, the switch absorbs the peer's anchor into
its own kernel bridge. The peer's interface (Docker veth host end, QEMU
TAP, IOU port TAP, Dynamips port TAP) becomes the switch port, and frames
cross anchor -> switch bridge -> switch ports entirely in the kernel.

Wire format: the switch end receives nio_anchor ('join this interface to
my own bridge' — the mirror of nio_bridge), the peer end nio_bridge with a
null bridge ('your anchor is bridged elsewhere': carrier, markers and
impairments only — never bridge membership, and never a bridge delete, on
an interface this node does not own). Anchor names come from the shared
naming contract (utils.kernel_anchor), a pure function of node type, id,
adapter and port — the controller computes the peer's anchor while the
node may be stopped, which is what makes links to stopped nodes wireable.

Stopped peers defer the join; after any node of a link starts
(UDPLink.node_started) the controller re-pushes the switch NIO, and the
switch completes — or repairs, when a peer restart replaced its interface
under the same name — the join against the kernel's actual membership
instead of assuming it.

VLAN changes now edit membership in place: only the ports whose mode/VLAN
actually changed are touched, each losing exactly the previous mode's
entries the new mode does not want (the dot1q admits-all entry is one
range operation in both directions), and no port ever leaves the bridge to
change its VLANs. This replaces the old 'reset by re-enslaving' shortcut
(delif+addif on every wired port, added when the ports were the switch's
own TAPs): on absorbed anchors, re-enslaving an interface owned by another
node is where live testing watched links die — anchors occasionally lost
their bridge membership right after a ports_mapping update (later updates
then failed with delif EINVAL), reproducibly with qemu, IOU and Dynamips
peers and never with Docker's veths.

Eligibility: the switch side needs no capability of its own (its bridge
exists by construction); the peer must be able to anchor. Switch-to-switch
cascades stay on the relay — a kernel interface belongs to one bridge, and
joining two bridges needs a veth pair, deferred.

Verified live on a real server: qemu peers through four VLAN cycles with
both anchors enslaved throughout; a mixed switch (2 IOU + 2 Dynamips, four
kernel ports); Docker peers with real ICMP isolating and recovering (with
both anchors still enslaved); VPCS relay ports likewise in place.

gns3server/controller/link.py

@@ -202,13 +202,16 @@ class Link:
     @property
     def kernel_datapath(self):
         """
-        Whether this link is wired on the kernel datapath (veth pairs
-        enslaved into a per-link Linux bridge — no uBridge relay in the
-        forwarding path). Impairment filters run as tc netem on the veth
-        host end; markers and capture are served by uBridge's AF_PACKET
+        Whether this link is wired on the kernel datapath (no uBridge relay
+        in the forwarding path): either both endpoints enslave their
+        anchors into a per-link Linux bridge (``nio_bridge``), or an
+        Ethernet switch absorbs the peer's anchor into its own kernel
+        bridge (``nio_anchor`` on the switch end, ``nio_bridge`` with no
+        bridge on the peer end). Impairment filters run as tc netem on the
+        anchors; markers and capture are served by uBridge's AF_PACKET
         module.
         """
-        return any(d.get("type") == "nio_bridge" for d in (getattr(self, "_link_data", None) or []))
+        return any(d.get("type") in ("nio_bridge", "nio_anchor") for d in (getattr(self, "_link_data", None) or []))
 
     @property
     def markers(self):
@@ -335,9 +338,7 @@ class Link:
                 raise ControllerError(
                     "Packet filter(s) {} only run on a kernel-datapath link (tc netem on the "
                     "veth host end); this link is wired on the uBridge relay — delete and "
-                    "recreate the link to switch it to the kernel datapath".format(
-                        ", ".join(sorted(conflicts))
-                    )
+                    "recreate the link to switch it to the kernel datapath".format(", ".join(sorted(conflicts)))
                 )
 
         # An unchanged filters dict is normally a no-op skip, but a PUT that
@@ -685,6 +686,14 @@ class Link:
         """
         raise NotImplementedError
 
+    async def node_started(self, node):
+        """
+        Called after a node member of the link reached the started state.
+        The base link has nothing to re-synchronise; UDPLink uses it for
+        the Ethernet-switch fast path (the switch must re-join a peer anchor
+        that only comes into existence when the peer starts).
+        """
+
     def default_capture_file_name(self):
         """
         :returns: File name for a capture on this link