feat(docker): expose the eBPF time-window filter on kernel links

uBridge's window mode gained the semantics the B.2 spec asked for —
`tc window_drop <if> <start> <outage> <pct> [<period> [<jitter>]]`: a
single outage that traffic passes before *and* after, recurring flaps
with period >= outage, and a per-cycle jitter — so the kernel-only
`window_drop` filter type is now exposed:

- validation mirrors the tc grammar (3-5 params, ranges, period >=
  outage, jitter cap) and adds window_drop to KERNEL_ONLY_FILTERS;
  filter_inactive_filters keeps start=0 — unlike every other filter a
  zero first parameter means "the outage starts now", not "disabled".
- `_ubridge_apply_ebpf_drops` sends `tc window_drop` (positional
  pass-through) and turns the mode off explicitly when absent, under the
  same ebpf=1 gate.
- FILTERS entry (Start/Outage/Chance/Period/Jitter) + available_filters.

The schedule is relative to the apply: reconcile re-sets the classifier
on every apply, so any filter change, node restart or link reset restarts
the window.

Verified on a two-container kernel link, 15/15: a single outage
[1000, 1500, 100] arrives per-seq as [0..4, 13..19] (passing before the
window, dropped inside, passing after); recurring 1000/2000 ms = 52 %
round trip (duty cycle); period == outage = permanent outage; period <
outage = 409 before uBridge; jittered flap keeps the ~50 % duty cycle;
30 % chance on a permanent window = 53 % round trip against the
per-direction theory 1-0.7^2 = 51 %; coexist with netem; node restart
restores; available_filters kernel vs relay (409 + hidden); teardown
clean. P6b re-run as a regression on the same uBridge build: 13/13.

Worth recording for the next rebuild: the first delivered window program
loaded as root but reported ebpf=0 through gns3-server — the verifier's
non-root path loses the loop counter's bound, treats the catch-up loop as
unbounded and rejects the *whole* program (E2BIG, 8193 pending states),
which also took frequency_drop and quota down, with ebpf=0 as the only
symptom. Smaller trip caps did not help; the loop had to go (uBridge
572ca0c), on top of a Makefile header-dependency fix (53e34bb) that had
been shipping a stale program object.

tests/compute/docker/test_docker_kernel_datapath.py

@@ -497,6 +497,7 @@ async def test_update_nio_kernel_turns_ebpf_modes_off(vm):
 
     vm._ubridge_send.assert_any_call(f'tc nth_drop "{host_ifc}" off')
     vm._ubridge_send.assert_any_call(f'tc quota_drop "{host_ifc}" off')
+    vm._ubridge_send.assert_any_call(f'tc window_drop "{host_ifc}" off')
     for c in vm._ubridge_send.call_args_list:
         assert "nth_drop" not in c.args[0] or "off" in c.args[0]
 
@@ -520,6 +521,54 @@ async def test_apply_ebpf_drops_requires_ebpf_capability(vm):
     vm._ubridge_send.assert_not_called()
 
 
+@pytest.mark.asyncio
+async def test_update_nio_kernel_applies_window_drop(vm):
+    """
+    window_drop becomes the eBPF time-window mode: the positional
+    [start, outage, chance, period, jitter] maps 1:1 onto the tc command
+    (trailing optionals omitted), and an absent window is turned off.
+    """
+
+    vm._ubridge_hypervisor = _caps_hypervisor(ebpf="1")
+    vm._ubridge_send = AsyncioMagicMock()
+    vm.status = "started"
+    host_ifc, _ = vm._veth_names(0, 0)
+    vm._kernel_veths[(0, 0)] = host_ifc
+    nio = vm.manager.create_nio({"type": "nio_bridge", "bridge": BRIDGE})
+    vm._ethernet_adapters[0].add_nio(0, nio)
+
+    nio.filters = {"window_drop": [0, 2000, 100]}
+    await vm.adapter_update_nio_binding(0, nio)
+    vm._ubridge_send.assert_any_call(f'tc window_drop "{host_ifc}" 0 2000 100')
+
+    vm._ubridge_send.reset_mock()
+    nio.filters = {"window_drop": [500, 800, 100, 2400, 300]}
+    await vm.adapter_update_nio_binding(0, nio)
+    vm._ubridge_send.assert_any_call(f'tc window_drop "{host_ifc}" 500 800 100 2400 300')
+
+    # only the window mode present: nth/quota are explicitly off
+    vm._ubridge_send.assert_any_call(f'tc nth_drop "{host_ifc}" off')
+    vm._ubridge_send.assert_any_call(f'tc quota_drop "{host_ifc}" off')
+
+
+@pytest.mark.asyncio
+async def test_apply_window_drop_requires_ebpf_capability(vm):
+    """
+    The window mode shares the eBPF capability gate (error names setcap).
+    """
+
+    hyp = MagicMock()
+    hyp.is_running.return_value = True
+    hyp.send = AsyncioMagicMock(return_value=["netem=delay;ebpf=0;cbpf=1"])
+    vm._ubridge_hypervisor = hyp
+    vm._ubridge_send = AsyncioMagicMock()
+    host_ifc, _ = vm._veth_names(0, 0)
+
+    with pytest.raises(DockerError, match="setcap cap_bpf"):
+        await vm._ubridge_apply_ebpf_drops(host_ifc, {"window_drop": [0, 2000, 100]})
+    vm._ubridge_send.assert_not_called()
+
+
 @pytest.mark.asyncio
 async def test_apply_ebpf_drops_skips_old_ubridge_without_filters(vm):
     """