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.

gns3server/compute/docker/docker_vm.py

@@ -1950,7 +1950,8 @@ class DockerVM(BaseNode):
             # Impairment filters become one tc netem qdisc on the veth host
             # end (restored here on node restart, like the capture above);
             # bpf expressions become cls_bpf match-drop classifiers and
-            # frequency_drop/quota become the eBPF stateful classifier.
+            # frequency_drop/quota/window_drop become the eBPF stateful
+            # classifier.
             await self._ubridge_apply_netem(host_ifc, nio.filters)
             await self._ubridge_apply_bpf_drops(host_ifc, nio.filters)
             await self._ubridge_apply_ebpf_drops(host_ifc, nio.filters)
@@ -2379,20 +2380,28 @@ class DockerVM(BaseNode):
         Translate the NIO's stateful filters into uBridge's eBPF impairment
         classifier on the veth host end's clsact egress (prio 1, below
         bpf_drop's 10-99): ``frequency_drop`` becomes the exact every-Nth
-        mode (``tc nth_drop``; -1 = drop everything maps to every 1st), and
-        the kernel-only ``quota`` type the byte-cap mode (``tc quota_drop``).
+        mode (``tc nth_drop``; -1 = drop everything maps to every 1st), the
+        kernel-only ``quota`` type the byte-cap mode (``tc quota_drop``),
+        and the kernel-only ``window_drop`` type the time-window mode
+        (``tc window_drop``: single outage, or recurring flaps with an
+        optional per-cycle jitter). The program evaluates the modes in the
+        fixed order nth → quota → window.
 
         Reconcile is a plain re-set on every apply: the preceding netem
         apply resets the interface (which detaches the program and drops
         uBridge's per-interface registry entry), so a mode set re-loads the
         program from scratch, and a mode left absent is turned off
-        explicitly (idempotent) so a removed filter stops dropping.
+        explicitly (idempotent) so a removed filter stops dropping. For
+        window_drop this also means the schedule restarts on every apply:
+        the first window opens Start-ms from the moment of this call.
 
         Semantic difference vs the relay, documented server-side: the relay
         counts packets of BOTH directions through its single filtered
         bridge, while each veth end's classifier counts only the traffic
         entering that container — with both endpoints applying theirs, each
-        direction drops every Nth independently.
+        direction drops every Nth independently (their window schedules
+        start ~simultaneously, so a round trip survives only when both
+        directions are outside their windows).
 
         :param host_ifc: veth host-end interface name
         :param filters: NIO filters dictionary
@@ -2406,24 +2415,28 @@ class DockerVM(BaseNode):
 
         frequency = values_of("frequency_drop")
         quota = values_of("quota")
-        if not frequency and not quota and self._ubridge_tc_caps is None:
+        window = values_of("window_drop")
+        if not frequency and not quota and not window and self._ubridge_tc_caps is None:
             # Nothing to set and nothing was ever installed (no probe yet) —
             # skip touching an old uBridge entirely.
             return
         caps = await self._ubridge_tc_capabilities()
-        if not frequency and not quota:
+        if not frequency and not quota and not window:
             if caps.get("ebpf") == "1":
                 # Only when the eBPF module answers can anything be
                 # installed; "off" is idempotent in uBridge.
                 await self._ubridge_send(f'tc nth_drop "{host_ifc}" off')
                 await self._ubridge_send(f'tc quota_drop "{host_ifc}" off')
+                await self._ubridge_send(f'tc window_drop "{host_ifc}" off')
             return
         if caps.get("ebpf") != "1":
             raise DockerError(
                 "Packet filter(s) {} on a kernel-datapath link need a uBridge with eBPF support "
                 "(tc capabilities reports no ebpf; setcap cap_bpf,cap_net_admin,cap_net_raw=ep on "
                 "the uBridge binary); upgrade uBridge on this compute or keep the link on the "
-                "relay datapath".format(", ".join(sorted(f for f in ("frequency_drop", "quota") if f in filters)))
+                "relay datapath".format(
+                    ", ".join(sorted(f for f in ("frequency_drop", "quota", "window_drop") if f in filters))
+                )
             )
         if frequency:
             # relay semantics: -1 = drop everything, N = every Nth packet
@@ -2437,6 +2450,13 @@ class DockerVM(BaseNode):
             )
         else:
             await self._ubridge_send(f'tc quota_drop "{host_ifc}" off')
+        if window:
+            # [start, outage, chance, period?, jitter?] — start is relative
+            # to this apply (the schedule restarts on every reconcile).
+            params = " ".join(str(int(v)) for v in window)
+            await self._ubridge_send(f'tc window_drop "{host_ifc}" {params}')
+        else:
+            await self._ubridge_send(f'tc window_drop "{host_ifc}" off')
 
     async def _remove_kernel_markers(self, host_ifc):
         """