Add e2e latency consistency test for WireGuard tunnel

Sends 50 pings at 0.2s intervals through the cell-to-cell tunnel and
asserts that ≤5% exceed 3× the median RTT (floor 15ms). Catches
server-side packet processing regressions on wired paths.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

tests/e2e/wg/test_cell_to_cell_routing.py

@@ -401,6 +401,33 @@ class TestCellToCellRouting:
                 f'Cell1 web (Caddy) at {url} not reachable via tunnel: {e}'
             )
 
+    def test_tunnel_latency_consistency(self, wg_setup):
+        """WireGuard tunnel has no significant latency spikes on a local wired network.
+
+        Sends 50 pings at 0.2s intervals to cell2's WG server IP. Pass condition:
+        ≤ 5% of pings (≤ 2 out of 50) exceed max(3× median RTT, 15ms).
+        """
+        cell2_wg_ip = wg_setup['cell2_wg_ip']
+        r = _run(['ping', '-c', '50', '-i', '0.2', '-W', '2', cell2_wg_ip], timeout=25)
+        assert r.returncode == 0, f'All pings to {cell2_wg_ip} failed: {r.stderr}'
+
+        rtts = [float(m.group(1)) for m in re.finditer(r'time=([\d.]+) ms', r.stdout)]
+        assert len(rtts) >= 40, (
+            f'Too few ping replies ({len(rtts)}/50) — packet loss may mask latency issues'
+        )
+
+        sorted_rtts = sorted(rtts)
+        median = sorted_rtts[len(sorted_rtts) // 2]
+        spike_threshold = max(median * 3.0, 15.0)
+        spikes = [rtt for rtt in rtts if rtt > spike_threshold]
+        spike_ratio = len(spikes) / len(rtts)
+
+        assert spike_ratio <= 0.05, (
+            f'Latency spikes: {len(spikes)}/{len(rtts)} packets ({spike_ratio:.0%}) '
+            f'exceeded {spike_threshold:.1f}ms (3× median {median:.1f}ms). '
+            f'Spike values: {[f"{s:.1f}ms" for s in sorted(spikes)]}'
+        )
+
     def test_cross_cell_domain_accessible(self, wg_setup):
         """A service domain from cell1 is resolvable via cell2's DNS and HTTP-reachable.