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, 8 insertions, 5 deletions

diff --git a/internal/tui/tui_test.go b/internal/tui/tui_test.go
index 5f007e4..d76ccde 100644
--- a/internal/tui/tui_test.go
+++ b/internal/tui/tui_test.go
@@ -459,8 +459,8 @@ type fakeDashboardSource struct {
 	snap *statsengine.Snapshot
 }
 
-func (f fakeDashboardSource) Snapshot() *statsengine.Snapshot {
-	return f.snap
+func (f fakeDashboardSource) Snapshot() (*statsengine.Snapshot, error) {
+	return f.snap, nil
 }
 
 type fakeResettableDashboardSource struct {
@@ -468,8 +468,8 @@ type fakeResettableDashboardSource struct {
 	resetCalls int
 }
 
-func (f *fakeResettableDashboardSource) Snapshot() *statsengine.Snapshot {
-	return f.snap
+func (f *fakeResettableDashboardSource) Snapshot() (*statsengine.Snapshot, error) {
+	return f.snap, nil
 }
 
 func (f *fakeResettableDashboardSource) Reset() {
@@ -484,7 +484,10 @@ func TestDashboardRefreshPicksLateBoundSource(t *testing.T) {
 	want := &statsengine.Snapshot{TotalSyscalls: 77}
 	runtime.SetDashboardSnapshotSource(fakeDashboardSource{snap: want})
 
-	got := source.Snapshot()
+	got, err := source.Snapshot()
+	if err != nil {
+		t.Fatalf("unexpected snapshot error: %v", err)
+	}
 	if got != want {
 		t.Fatalf("expected late-bound source to use latest runtime source")
 	}