feat(docker): eBPF stateful packet filters on kernel links

Consume the eBPF impairment classifier delivered in uBridge
feature/tc-precision: frequency_drop becomes the exact every-Nth mode
(tc nth_drop; -1 = drop everything maps to every 1st) and the new
kernel-only quota type is the byte-cap mode (tc quota_drop, 100 = hard
cutoff). KERNEL_UNSUPPORTED_FILTERS is gone entirely — kernel/relay
eligibility is now purely topological. Both modes gate on ebpf=1 from
the per-process tc capabilities probe (error names the setcap fix); the
cleanup path (modes off) only talks to the eBPF commands when the probe
reported them. Reconcile is a plain re-set: the netem reset-before-set
already detaches the program and clears uBridge's per-interface
registry, so a mode set after it is a fresh load+attach.

Documented semantic difference: the relay counts both directions
through one bridge counter, the kernel counts per direction — N=3
round-trip loss is 1-(2/3)^2 = 55.6% on kernel vs ~1/N on relay
(measured 57%). window_drop is NOT exposed: the delivered
implementation advances the window with no gap, making 'outside the
window packets pass' unreachable (needs outage/period fields —
recorded in the spec); flow_drop awaits a parameter-shape decision.

Also fixes a pre-existing restart bug the e2e exposed: _bridges (relay
bridge names) was never cleared on stop, so restarting a node with a
relay link skipped 'bridge create' and failed on add_nio_udp.

e2e (isolated instance): 13/13 — pref-1 tc_impair classifier on both
veths, 57% round-trip for N=3, -1 = exact 100%, quota hard cutoff,
delay coexistence, clean teardown, suspend/resume, node restart,
relay-leg frequency_drop regression through an Ethernet switch. Full
suite: 1925 passed.

tests/utils/test_packet_filter_validation.py

@@ -366,3 +366,20 @@ class TestNetemExtensionFilters:
     def test_kernel_only_features_empty(self):
         assert split_kernel_only_features({}) == ({}, set())
         assert split_kernel_only_features(None) == ({}, set())
+
+    def test_quota_valid_and_invalid(self):
+        validate_filter_parameters("quota", [1000000, 100])
+        validate_filter_parameters("quota", [1, 0])
+        with pytest.raises(FilterValidationError, match="Quota"):
+            validate_filter_parameters("quota", [0, 100])
+        with pytest.raises(FilterValidationError, match="Chance"):
+            validate_filter_parameters("quota", [1000, 101])
+        with pytest.raises(FilterValidationError, match="expects 2 parameter"):
+            validate_filter_parameters("quota", [1000])
+
+    def test_quota_is_kernel_only(self):
+        clean, dropped = split_kernel_only_features({"quota": [1000, 50], "frequency_drop": [7]})
+        assert dropped == {"quota"}
+        # frequency_drop runs on both datapaths (relay userspace filter /
+        # eBPF every-Nth) and stays
+        assert clean == {"frequency_drop": [7]}