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, 12 insertions, 9 deletions

diff --git a/internal/statsengine/snapshot.go b/internal/statsengine/snapshot.go
index f2b617b..7a95ab8 100644
--- a/internal/statsengine/snapshot.go
+++ b/internal/statsengine/snapshot.go
@@ -68,12 +68,13 @@ type SyscallSnapshot struct {
 	Errors     uint64
 	Bytes      uint64
 
-	LatencyMinNs  uint64
-	LatencyMaxNs  uint64
-	LatencyMeanNs float64
-	LatencyP50Ns  uint64
-	LatencyP95Ns  uint64
-	LatencyP99Ns  uint64
+	LatencyMinNs   uint64
+	LatencyMaxNs   uint64
+	LatencyMeanNs  float64
+	TotalLatencyNs uint64
+	LatencyP50Ns   uint64
+	LatencyP95Ns   uint64
+	LatencyP99Ns   uint64
 }
 
 // FileSnapshot is an aggregated per-file ranking entry.
@@ -84,8 +85,9 @@ type FileSnapshot struct {
 	BytesRead    uint64
 	BytesWritten uint64
 
-	AvgLatencyNs float64
-	MaxLatencyNs uint64
+	AvgLatencyNs   float64
+	MaxLatencyNs   uint64
+	TotalLatencyNs uint64
 }
 
 // ProcessSnapshot is an aggregated per-process entry.
@@ -97,7 +99,8 @@ type ProcessSnapshot struct {
 	RatePerSec float64
 	Bytes      uint64
 
-	AvgLatencyNs float64
+	AvgLatencyNs   float64
+	TotalLatencyNs uint64
 }
 
 // HistogramBucketSnapshot is one bucket of a histogram snapshot.