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/utils/test_packet_filter_validation.py

@@ -383,3 +383,36 @@ class TestNetemExtensionFilters:
         # frequency_drop runs on both datapaths (relay userspace filter /
         # eBPF every-Nth) and stays
         assert clean == {"frequency_drop": [7]}
+
+    def test_window_drop_valid_and_invalid(self):
+        # the three delivered forms: single outage, recurring, jittered
+        validate_filter_parameters("window_drop", [0, 2000, 100])
+        validate_filter_parameters("window_drop", [500, 800, 100, 2400])
+        validate_filter_parameters("window_drop", [500, 800, 100, 2400, 300])
+        with pytest.raises(FilterValidationError, match="expects 3 to 5"):
+            validate_filter_parameters("window_drop", [500, 800])
+        with pytest.raises(FilterValidationError, match="expects 3 to 5"):
+            validate_filter_parameters("window_drop", [500, 800, 100, 2400, 300, 1])
+        with pytest.raises(FilterValidationError, match="Outage"):
+            validate_filter_parameters("window_drop", [0, 0, 100])
+        with pytest.raises(FilterValidationError, match="Chance"):
+            validate_filter_parameters("window_drop", [0, 2000, 101])
+        with pytest.raises(FilterValidationError, match="Jitter"):
+            validate_filter_parameters("window_drop", [0, 2000, 100, 2400, 10**10])
+
+    def test_window_drop_is_kernel_only(self):
+        clean, dropped = split_kernel_only_features({"window_drop": [0, 800, 100, 2400], "delay": [10, 0]})
+        assert dropped == {"window_drop"}
+        assert clean == {"delay": [10, 0]}
+
+    def test_window_drop_period_must_cover_outage(self):
+        with pytest.raises(FilterValidationError, match="period must be greater than or equal"):
+            validate_all_filters({"window_drop": [0, 2000, 100, 1000]})
+        # period == outage is accepted (uBridge allows it); shorter is not
+        validate_all_filters({"window_drop": [0, 2000, 100, 2000]})
+
+    def test_window_drop_start_zero_is_active(self):
+        # start=0 means "the outage starts now" — unlike other filters a
+        # zero first parameter must NOT be treated as "disabled"
+        active = filter_inactive_filters({"window_drop": [0, 2000, 100]})
+        assert active == {"window_drop": [0, 2000, 100]}