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_reset_test.go b/internal/statsengine/engine_reset_test.go
index e798686..c09c059 100644
--- a/internal/statsengine/engine_reset_test.go
+++ b/internal/statsengine/engine_reset_test.go
@@ -10,13 +10,19 @@ import (
 func TestEngineResetClearsAccumulatedStats(t *testing.T) {
 	e := NewEngine(8)
 	e.Ingest(newEnginePair(types.SYS_ENTER_READ, 7, types.READ_CLASSIFIED, "test", 1, "/tmp/a", 7, 1000, 50))
-	before := e.Snapshot()
+	before, err := e.Snapshot()
+	if err != nil {
+		t.Fatalf("unexpected snapshot error: %v", err)
+	}
 	if before.TotalSyscalls == 0 {
 		t.Fatalf("expected non-zero totals before reset")
 	}
 
 	e.Reset()
-	after := e.Snapshot()
+	after, err := e.Snapshot()
+	if err != nil {
+		t.Fatalf("unexpected snapshot error after reset: %v", err)
+	}
 	if after.TotalSyscalls != 0 || after.TotalBytes != 0 || after.TotalErrors != 0 {
 		t.Fatalf("expected totals cleared after reset, got %+v", after)
 	}