feat(esw): cascade two switches through a veth pair

Switch-to-switch links were the last shape left on the relay: the switch
fast path works by absorbing the *peer's* anchor, and a switch has none —
its ports are its own bridge, and a kernel interface belongs to exactly
one bridge, so two bridges cannot be joined by enslaving anything.

Join them with a veth pair instead: one end per bridge, each side
enslaving its own end and applying its own port mode to it — an access
port on one side and a trunk on the other behaves like the real cable it
models. uBridge's docker create_veth/delete_veth (the primitive every
Docker adapter is already born from) creates and destroys the pair; the
names derive from the link id (utils.kernel_anchor.kernel_cascade_names),
so each side computes its own end and the names survive a restart.

The pair belongs to the link, and the two ends are wired by two
independent uBridge processes — the project-open batch drives different
nodes concurrently — so the protocol is order-free: both sides create the
pair when their own end is missing (one create wins; the loser's failure
is re-checked against the kernel and "the pair is there" is not an
error), and either side's teardown destroys it (deleting one veth end
takes the other with it, so both converge on the same gone pair and the
second delif answers ENOENT). A pre-existing end — this switch restarted,
or a leftover from a previous life of the link id — is reused rather than
recreated, because the other end may still be a live port of the peer
switch; a stale tc qdisc is all a reused end can carry.

No new wire format was needed: a cascade is an anchor NIO on both ends,
the absorbed-anchor case plus the peer end's name. That also corrects the
filter routing — the single-sided rule exists because an absorbed anchor
is one interface, while a cascade has one per switch, so impairments go
to both ends like any other kernel link.

Tests: the wire format, the eligibility flip and the both-ends re-push on
node start (controller); the naming contract (utils); the switch-side
lifecycle — create, reuse, lost create race, delete destroys the pair
(compute routes).

Drive-by: the relay filter test's compute mock carried no awaitable
`put`, so tests/controller/test_kernel_datapath_link.py failed when run
alone while an argument-less full run passed; callers now get one.

tests/utils/test_kernel_anchor.py

@@ -17,12 +17,14 @@
 """
 The kernel-datapath anchor naming contract (shared by the compute classes
 that create anchors and the controller that names a peer's anchor for an
-Ethernet-switch link).
+Ethernet-switch link), plus the cascade veth naming for switch-to-switch
+links.
 """
 
-from gns3server.utils.kernel_anchor import anchor_suffix, kernel_anchor_name
+from gns3server.utils.kernel_anchor import anchor_suffix, kernel_anchor_name, kernel_cascade_names
 
 NODE_ID = "00010203-0405-0607-0809-0a0b0c0d0e0f"
+LINK_ID = "1a2b3c4d-5e6f-4a5b-8c9d-0e1f2a3b4c5d"
 
 
 def test_every_node_type_has_its_own_prefix():
@@ -45,3 +47,22 @@ def test_names_fit_ifnamsiz():
 def test_suffix_is_stable_across_the_node_type():
     assert anchor_suffix(NODE_ID, 1, 2) == "00010203e1p2"
     assert kernel_anchor_name("docker", NODE_ID, 1, 2) == "gv" + anchor_suffix(NODE_ID, 1, 2)
+
+
+def test_cascade_names_are_a_pair_of_distinct_sides():
+    end0, end1 = kernel_cascade_names(LINK_ID)
+    assert end0 == "gs1a2b3c4d5e0"
+    assert end1 == "gs1a2b3c4d5e1"
+    assert end0 != end1
+
+
+def test_cascade_names_fit_ifnamsiz():
+    for end in kernel_cascade_names(LINK_ID):
+        assert len(end) <= 15
+
+
+def test_cascade_names_stay_out_of_the_node_anchor_namespace():
+    # "gs" cannot collide with any node anchor prefix (gv/gq/gi/gd), so a
+    # cascade end is never mistaken for another node's anchor
+    for end in kernel_cascade_names(LINK_ID):
+        assert not any(end.startswith(prefix) for prefix in ("gv", "gq", "gi", "gd"))