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, 7 insertions, 6 deletions

diff --git a/internal/statsengine/filerank.go b/internal/statsengine/filerank.go
index d24ab93..ef43c6e 100644
--- a/internal/statsengine/filerank.go
+++ b/internal/statsengine/filerank.go
@@ -191,12 +191,13 @@ func (s fileSnapshotInput) toSnapshot() FileSnapshot {
 	}
 
 	return FileSnapshot{
-		Path:         s.path,
-		Accesses:     s.accesses,
-		BytesRead:    s.bytesRead,
-		BytesWritten: s.bytesWritten,
-		AvgLatencyNs: avg,
-		MaxLatencyNs: s.maxLatency,
+		Path:           s.path,
+		Accesses:       s.accesses,
+		BytesRead:      s.bytesRead,
+		BytesWritten:   s.bytesWritten,
+		AvgLatencyNs:   avg,
+		MaxLatencyNs:   s.maxLatency,
+		TotalLatencyNs: s.totalLatency,
 	}
 }