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, 11 insertions, 2 deletions

diff --git a/internal/tui/dashboard/model.go b/internal/tui/dashboard/model.go
index 8a3c5d4..cc2a052 100644
--- a/internal/tui/dashboard/model.go
+++ b/internal/tui/dashboard/model.go
@@ -874,11 +874,18 @@ func (m Model) maxProcessesRows() int {
 	return m.snapshotOrZero().ProcessesCount()
 }
 
+// snapshot returns the latest engine snapshot, or nil if the engine is nil or
+// returns an error. Errors are silently dropped here because the dashboard
+// renders the last successful snapshot on transient failures.
 func (m Model) snapshot() *statsengine.Snapshot {
 	if m.engine == nil {
 		return nil
 	}
-	return m.engine.Snapshot()
+	snap, err := m.engine.Snapshot()
+	if err != nil {
+		return nil
+	}
+	return snap
 }
 
 func (m Model) snapshotOrZero() statsengine.Snapshot {
@@ -893,10 +900,12 @@ func (m *Model) resetBaselineCmd() tea.Cmd {
 		m.liveTrie.Reset()
 	}
 
+	// Errors from Snapshot are silently dropped here; the dashboard will
+	// continue to display the last successful snapshot.
 	var snap *statsengine.Snapshot
 	if resettable, ok := m.engine.(resettableSnapshotSource); ok {
 		resettable.Reset()
-		snap = resettable.Snapshot()
+		snap, _ = resettable.Snapshot()
 	} else {
 		snap = m.snapshot()
 	}