gpu topology: detect cross-NUMA-only paths between GPU pairs

Flags GPUs that reach one or more peers only via a SYS-class PCIe hop
(crossing the CPU/NUMA-node boundary) in nvidia-smi topo -m. On servers
where GPUs are only bridged pairwise via NVLink bridge (no switched
NVLink fabric), this is the exact path that traffic between different
bridge pairs has to cross, and can cut multi-GPU throughput by 2x+ for
workloads spanning more than one pair.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

audit/internal/collector/pcie_nvlink_bridge_test.go

@@ -80,6 +80,42 @@ func TestParseNVIDIATopologyMatrixEmpty(t *testing.T) {
 	}
 }
 
+func TestParseCrossNUMAPeersDetectsSYS(t *testing.T) {
+	t.Parallel()
+
+	// 4-GPU box, two NVLink-bridged pairs (GPU0-GPU1, GPU2-GPU3); the pairs
+	// themselves only reach each other via SYS (cross-NUMA PCIe hop) — the
+	// exact topology of a server using pairwise NVLink bridge cards instead
+	// of a switched NVLink fabric.
+	input := `	GPU0	GPU1	GPU2	GPU3
+GPU0	 X 	NV4	SYS	SYS
+GPU1	NV4	 X 	SYS	SYS
+GPU2	SYS	SYS	 X 	NV4
+GPU3	SYS	SYS	NV4	 X
+`
+	peers := parseCrossNUMAPeers(input)
+
+	if len(peers[0]) != 2 || peers[0][0] != 2 || peers[0][1] != 3 {
+		t.Fatalf("peers[0]=%v want [2 3]", peers[0])
+	}
+	if len(peers[2]) != 2 {
+		t.Fatalf("peers[2]=%v want 2 entries", peers[2])
+	}
+}
+
+func TestParseCrossNUMAPeersNoSYS(t *testing.T) {
+	t.Parallel()
+
+	// Full NVSwitch fabric: every GPU pair connects via NVLink, no SYS hops.
+	input := `	GPU0	GPU1
+GPU0	 X 	NV18
+GPU1	NV18	 X
+`
+	if peers := parseCrossNUMAPeers(input); peers != nil {
+		t.Fatalf("peers=%v want nil (no SYS pairs)", peers)
+	}
+}
+
 func TestApplyPCIeLinkSpeedWarningNVLinkBridgeEscalates(t *testing.T) {
 	t.Parallel()