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/syscall.go b/internal/statsengine/syscall.go
index 2ef929a..d58e8c9 100644
--- a/internal/statsengine/syscall.go
+++ b/internal/statsengine/syscall.go
@@ -99,12 +99,18 @@ func (a *syscallAccumulator) Add(pair *event.Pair) {
 	}
 }
 
+// Snapshot returns a slice of SyscallSnapshots for all tracked syscalls.
+// It panics on build error, which should never happen for a valid accumulator.
 func (a *syscallAccumulator) Snapshot(elapsed time.Duration) []SyscallSnapshot {
 	if a == nil {
 		return nil
 	}
 
-	return buildSyscallSnapshots(a.snapshotInputs(), elapsed)
+	snap, err := buildSyscallSnapshots(a.snapshotInputs(), elapsed)
+	if err != nil {
+		panic("buildSyscallSnapshots: " + err.Error())
+	}
+	return snap
 }
 
 func (a *syscallAccumulator) snapshotInputs() []syscallSnapshotInput {
@@ -132,7 +138,10 @@ func (a *syscallAccumulator) snapshotInputs() []syscallSnapshotInput {
 	return inputs
 }
 
-func buildSyscallSnapshots(inputs []syscallSnapshotInput, elapsed time.Duration) []SyscallSnapshot {
+// buildSyscallSnapshots converts raw syscall accumulator inputs into sorted
+// SyscallSnapshot slices. The error return is reserved for future validation;
+// currently this function always succeeds.
+func buildSyscallSnapshots(inputs []syscallSnapshotInput, elapsed time.Duration) ([]SyscallSnapshot, error) {
 	rateDiv := elapsed.Seconds()
 	result := make([]SyscallSnapshot, 0, len(inputs))
 	for _, in := range inputs {
@@ -144,7 +153,7 @@ func buildSyscallSnapshots(inputs []syscallSnapshotInput, elapsed time.Duration)
 		}
 		return cmp.Compare(a.Name, b.Name)
 	})
-	return result
+	return result, nil
 }
 
 func (s *syscallStats) updateMinMax(duration uint64) {