use errgroup instead of WaitGroup for concurrent snapshot builders

Replace sync.WaitGroup with errgroup.Group in buildSubSnapshots so errors from
sub-builders (buildSyscallSnapshots, buildFileSnapshots, buildProcessSnapshots,
buildHistogramSnapshot) are captured and propagated rather than silently dropped.

Change Engine.Snapshot() to return (*Snapshot, error), update runtime.SnapshotSource
and dashboard.SnapshotSource interfaces accordingly, and adjust all callers in
tui.go, dashboard/model.go, and the test helpers.

Each sub-builder now returns (result, error); the error return is currently
always nil but establishes the contract for future validation. The per-type
Snapshot() convenience methods (histogram, syscall, file, process) panic on
error since they are internal helpers where failure would be a programming bug.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

1 files changed, 12 insertions, 3 deletions

diff --git a/internal/statsengine/histogram.go b/internal/statsengine/histogram.go
index 42460ea..4a5e3b4 100644
--- a/internal/statsengine/histogram.go
+++ b/internal/statsengine/histogram.go
@@ -47,12 +47,18 @@ func (h *histogram) Increment(durationNs uint64) {
 	h.total++
 }
 
+// Snapshot returns a HistogramSnapshot of the current histogram state.
+// It panics on build error, which should never happen for a valid histogram.
 func (h *histogram) Snapshot() HistogramSnapshot {
 	if h == nil {
 		return NewHistogramSnapshot(0, nil)
 	}
 
-	return buildHistogramSnapshot(h.snapshotInputs())
+	snap, err := buildHistogramSnapshot(h.snapshotInputs())
+	if err != nil {
+		panic("buildHistogramSnapshot: " + err.Error())
+	}
+	return snap
 }
 
 func (h *histogram) snapshotInputs() histogramSnapshotInput {
@@ -65,7 +71,10 @@ func (h *histogram) snapshotInputs() histogramSnapshotInput {
 	}
 }
 
-func buildHistogramSnapshot(in histogramSnapshotInput) HistogramSnapshot {
+// buildHistogramSnapshot converts a histogramSnapshotInput into a
+// HistogramSnapshot. The error return is reserved for future validation;
+// currently this function always succeeds.
+func buildHistogramSnapshot(in histogramSnapshotInput) (HistogramSnapshot, error) {
 	buckets := make([]HistogramBucketSnapshot, 0, histogramBucketCount)
 	for i := 0; i < histogramBucketCount; i++ {
 		lower, upper := histogramBucketRange(i)
@@ -77,7 +86,7 @@ func buildHistogramSnapshot(in histogramSnapshotInput) HistogramSnapshot {
 		})
 	}
 
-	return NewHistogramSnapshot(in.total, buckets)
+	return NewHistogramSnapshot(in.total, buckets), nil
 }
 
 func histogramBucketIndex(durationNs uint64) int {