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, 2 deletions

diff --git a/internal/statsengine/engine_test.go b/internal/statsengine/engine_test.go
index 7ba8c3a..f714844 100644
--- a/internal/statsengine/engine_test.go
+++ b/internal/statsengine/engine_test.go
@@ -33,7 +33,10 @@ func TestEngineIngestAndSnapshotIntegration(t *testing.T) {
 	engine.Ingest(newEnginePair(types.SYS_ENTER_COPY_FILE_RANGE, 80, types.TRANSFER_CLASSIFIED, "proc-b", 2, "/tmp/b", 20, 40, 8))
 	clock.Advance(1 * time.Second)
 
-	snap := engine.Snapshot()
+	snap, err := engine.Snapshot()
+	if err != nil {
+		t.Fatalf("unexpected snapshot error: %v", err)
+	}
 	if snap == nil {
 		t.Fatalf("expected snapshot")
 	}
@@ -79,7 +82,10 @@ func TestEngineSnapshotWithNoEvents(t *testing.T) {
 	clock := &fakeClock{now: time.Unix(2000, 0)}
 	engine := newEngineWithClock(10, clock.Now)
 
-	snap := engine.Snapshot()
+	snap, err := engine.Snapshot()
+	if err != nil {
+		t.Fatalf("unexpected snapshot error: %v", err)
+	}
 	if snap == nil {
 		t.Fatalf("expected snapshot")
 	}