interpret: label a non-responding hop instead of printing None

a hop with no icmp reply has ip=None; the sustained-loss finding
formatted it straight into the report text as the literal string
'(None)'. seen live in a --quick smoke test report.

src/netcheck/interpret.py

@@ -125,7 +125,7 @@ def path_findings(trace: TraceResult) -> list[Finding]:
                     id="path.loss_jump",
                     severity="crit" if hop.loss_pct >= 50.0 else "warn",
                     title="Sustained loss starts mid-path",
-                    detail=f"Loss appears at hop {hop.ttl} ({hop.ip}) at {hop.loss_pct}% and persists downstream.",
+                    detail=f"Loss appears at hop {hop.ttl} ({hop.ip or 'no reply'}) at {hop.loss_pct}% and persists downstream.",
                     metric="loss_pct",
                     value=hop.loss_pct,
                     threshold=20.0,

tests/test_interpret.py

@@ -118,6 +118,24 @@ def test_path_findings_highlight_the_first_sustained_loss_jump():
     assert "198.51.100.7" in loss.detail
 
 
+def test_path_findings_label_a_non_responding_hop_instead_of_printing_none():
+    trace = TraceResult(
+        target="1.1.1.1",
+        resolved_ip="1.1.1.1",
+        backend="mtr_json",
+        hops=[
+            TraceHop(ttl=1, ip="192.168.1.1", loss_pct=0.0, avg_ms=1.0),
+            TraceHop(ttl=2, ip=None, loss_pct=100.0, avg_ms=None),
+            TraceHop(ttl=3, ip="1.1.1.1", loss_pct=100.0, avg_ms=None),
+        ],
+        completed=True,
+    )
+    findings = path_findings(trace)
+    loss = [f for f in findings if f.id == "path.loss_jump"][0]
+    assert "None" not in loss.detail
+    assert "no reply" in loss.detail
+
+
 def test_path_findings_ignore_a_single_hop_that_rate_limits_icmp():
     trace = TraceResult(
         target="1.1.1.1",