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.

tests/compute/dynamips/test_dynamips_kernel_datapath.py

@@ -612,3 +612,65 @@ async def test_manager_create_nio_builds_the_bridge_nio(router):
     assert nio.filters == {"delay": [5]}
     assert nio.suspend is True
     assert _hypervisor_commands(router) == []
+
+
+# ---------------------------------------------------------------------------
+# Externally bridged anchors (an Ethernet switch absorbed them)
+# ---------------------------------------------------------------------------
+
+
+@pytest.mark.asyncio
+async def test_external_bridge_nio_skips_bridge_membership(router):
+    """
+    A NIOBridge with no bridge means an Ethernet switch owns the anchor's
+    bridge membership: the router still opens the anchor in the hypervisor
+    (the TAP needs its fd holder) but never touches any kernel bridge.
+    """
+
+    router.status = "started"
+    router._kernel_taps[(0, 0)] = TAP00
+    nio = await _nio(router, bridge=None)
+
+    await router.slot_add_nio_binding(0, 0, nio)
+
+    hypervisor = _hypervisor_commands(router)
+    assert any(c.startswith("nio create_tap tap-") for c in hypervisor)
+    assert any(c.startswith('vm slot_add_nio_binding "R1" 0 0 tap-') for c in hypervisor)
+    assert not any("brctl" in c for c in _ubridge_commands(router))
+
+
+@pytest.mark.asyncio
+async def test_external_bridge_nio_remove_skips_bridge_ops(router):
+    """
+    Detaching an externally bridged anchor releases the hypervisor's fd and
+    resets the link state, but must not delif or delete any bridge — the
+    switch owns them, and deleting the switch's bridge would take the whole
+    switch down.
+    """
+
+    router.status = "started"
+    router._kernel_taps[(0, 0)] = TAP00
+    nio = await _nio(router, bridge=None)
+    await router.slot_add_nio_binding(0, 0, nio)
+    router._ubridge_send.reset_mock()
+
+    await router.slot_remove_nio_binding(0, 0)
+
+    commands = _ubridge_commands(router)
+    assert not any("brctl" in c for c in commands), commands
+    assert any(c == f'tc reset "{TAP00}"' for c in commands)
+
+
+@pytest.mark.asyncio
+async def test_remove_kernel_bridges_skips_external_nios(router):
+    """
+    The bridge sweep never deletes a bridge this node does not own.
+    """
+
+    router._kernel_taps[(0, 0)] = TAP00
+    external = await _nio(router, bridge=None)
+    router._slots[0].add_nio(0, external)
+
+    await router._remove_kernel_bridges()
+
+    assert not any("brctl" in c for c in _ubridge_commands(router))