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/compute/kernel_datapath.py

@@ -146,15 +146,23 @@ class KernelDatapathMixin:
         Both link endpoints run this concurrently and either may create the
         bridge first: an existing bridge is success instead of a create race
         failure.
+
+        A NIO whose ``bridge`` is None means the anchor is bridged
+        elsewhere — an Ethernet switch absorbed it into the switch's own
+        bridge — so the membership work is skipped here and only the link
+        state the anchor carries (capture, markers, impairment filters) is
+        applied. Exactly one end of such a link owns the anchor's bridge
+        state; the other end is a passive carrier.
         """
 
-        try:
-            await self._ubridge_send(f'brctl create "{nio.bridge}"')
-        except UbridgeError:
-            # Raises again if the bridge genuinely does not exist.
-            await self._ubridge_send(f'brctl show "{nio.bridge}"')
-        await self._ubridge_send(f'link set "{nio.bridge}" up')
-        await self._ubridge_send(f'brctl addif "{nio.bridge}" "{anchor}"')
+        if nio.bridge is not None:
+            try:
+                await self._ubridge_send(f'brctl create "{nio.bridge}"')
+            except UbridgeError:
+                # Raises again if the bridge genuinely does not exist.
+                await self._ubridge_send(f'brctl show "{nio.bridge}"')
+            await self._ubridge_send(f'link set "{nio.bridge}" up')
+            await self._ubridge_send(f'brctl addif "{nio.bridge}" "{anchor}"')
         if nio.capturing:
             # Restore a capture that was active before a node restart
             # (mirrors the relay path's start_capture).
@@ -235,6 +243,11 @@ class KernelDatapathMixin:
         this concurrently: deleting a bridge that still has the peer's port
         enslaved fails with EBUSY, so the last endpoint to remove its port
         wins the deletion and the loser's failure is expected and suppressed.
+
+        A NIO whose ``bridge`` is None owns no bridge here: the anchor is
+        bridged by an Ethernet switch, which tears its own membership down
+        on its side; this end only releases the link state it applied
+        (markers, impairments, carrier).
         """
 
         host_ifc = self._kernel_host_ifc(adapter_number, port_number)
@@ -248,10 +261,12 @@ class KernelDatapathMixin:
                 await self._ubridge_send(f'tc reset "{host_ifc}"')
             if self.status == "started":
                 await self._set_adapter_carrier(adapter_number, False, port_number)
+            if nio.bridge is not None:
+                with contextlib.suppress(UbridgeError):
+                    await self._ubridge_send(f'brctl delif "{nio.bridge}" "{host_ifc}"')
+        if nio.bridge is not None:
             with contextlib.suppress(UbridgeError):
-                await self._ubridge_send(f'brctl delif "{nio.bridge}" "{host_ifc}"')
-        with contextlib.suppress(UbridgeError):
-            await self._ubridge_send(f'brctl delete "{nio.bridge}"')
+                await self._ubridge_send(f'brctl delete "{nio.bridge}"')
 
     async def _remove_kernel_bridges(self):
         """
@@ -269,7 +284,9 @@ class KernelDatapathMixin:
             return
         for adapter in self._ethernet_adapters:
             for nio in adapter.ports.values():
-                if isinstance(nio, NIOBridge):
+                # bridge None = externally bridged (an Ethernet switch owns
+                # the bridge); never delete a bridge this node does not own.
+                if isinstance(nio, NIOBridge) and nio.bridge is not None:
                     with contextlib.suppress(UbridgeError):
                         await self._ubridge_send(f'brctl delete "{nio.bridge}"')