add duration metric, tolerate missing tracepoints, ship el8 build

- Bubbles, treemap, icicle, and the live flamegraph 'b' cycle now include
  syscall duration (sum) as a third metric alongside events and bytes.
  Statsengine snapshots expose TotalLatencyNs to support this.
- AttachAll takes an optional warn callback. Production passes one so older
  kernels that lack newer tracepoints log a warning and keep going instead
  of aborting startup.
- Dockerfile.el8 + scripts/build-with-docker-el8.sh + mage buildDockerEl8
  produce ior.el8, a static binary built against Rocky Linux 8 glibc for
  RHEL/Rocky/Alma 8 hosts.
- README.md documents installing mage and the new el8 target.

1 files changed, 13 insertions, 4 deletions

diff --git a/internal/tui/dashboard/model.go b/internal/tui/dashboard/model.go
index 850a483..42a9ad4 100644
--- a/internal/tui/dashboard/model.go
+++ b/internal/tui/dashboard/model.go
@@ -713,10 +713,14 @@ func sortedProcessSnapshots(rows []statsengine.ProcessSnapshot, metric bubbleMet
 }
 
 func processMetricValue(proc statsengine.ProcessSnapshot, metric bubbleMetric) uint64 {
-	if metric == bubbleMetricBytes {
+	switch metric {
+	case bubbleMetricBytes:
 		return proc.Bytes
+	case bubbleMetricDuration:
+		return proc.TotalLatencyNs
+	default:
+		return proc.Syscalls
 	}
-	return proc.Syscalls
 }
 
 func processSelectionLabel(proc statsengine.ProcessSnapshot) string {
@@ -1289,10 +1293,15 @@ func nextVizMode(current tabVizMode, allowed []tabVizMode) tabVizMode {
 }
 
 func nextBubbleMetric(metric bubbleMetric) bubbleMetric {
-	if metric == bubbleMetricBytes {
+	// 3-way cycle: count (events) → bytes → duration → count.
+	switch metric {
+	case bubbleMetricCount:
+		return bubbleMetricBytes
+	case bubbleMetricBytes:
+		return bubbleMetricDuration
+	default:
 		return bubbleMetricCount
 	}
-	return bubbleMetricBytes
 }
 
 func tickCmd(d time.Duration) tea.Cmd {