Monitor: show GPU temperature and fan speed

Extends the existing per-tick nvidia-smi query with temperature.gpu and
fan.speed (no extra call); N/A values (cards without fan telemetry) are
simply omitted from the GPU line.

internal/game/game_test.go

@@ -116,6 +116,31 @@ func TestParseGPUEngineInstance(t *testing.T) {
 	}
 }
 
+func TestParseGPUStats(t *testing.T) {
+	st, err := parseGPUStats("4300, 16384, 55, 42\n")
+	if err != nil {
+		t.Fatal(err)
+	}
+	if st.UsedMB != 4300 || st.TotalMB != 16384 || st.TempC != 55 || st.FanPct != 42 {
+		t.Errorf("got %+v", st)
+	}
+
+	// Cards that don't expose temperature/fan report N/A.
+	st, err = parseGPUStats("1024, 16384, N/A, N/A")
+	if err != nil {
+		t.Fatal(err)
+	}
+	if st.TempC != -1 || st.FanPct != -1 {
+		t.Errorf("got %+v, want -1 for N/A fields", st)
+	}
+
+	for _, bad := range []string{"", "1, 2", "x, 16384, 55, 42", "1024, x, 55, 42", "1, 2, 3, 4, 5"} {
+		if _, err := parseGPUStats(bad); err == nil {
+			t.Errorf("%q parsed, want failure", bad)
+		}
+	}
+}
+
 func TestProcessesLive(t *testing.T) {
 	if runtime.GOOS != "windows" && runtime.GOOS != "linux" {
 		t.Skip("no process listing on this platform")