Reconcile reliable execution documentation

engine.go

@@ -510,6 +510,11 @@ func (e *Engine) PrepareExecution(ctx context.Context, req RunRequest) (*Prepare
 // model attempts. Each dispatched attempt receives its own generation and
 // HTTP-client timeout; the caller context governs the full operation.
 // Injected LLMClient implementations are not retried automatically.
+// With Observer, Run emits started before facade request conversion or a nil
+// engine check, and finished on every normal return after public error mapping
+// and resource release. The event RunID matches a successful result. Without
+// Observer, result timing retains its existing convention; observed wall
+// duration can be slightly longer because it spans the facade boundary.
 //
 // Run can return every error category documented by [Engine.Prepare], plus
 // ErrCapacityExceeded and ErrLLMGenerate. Built-in HTTP execution failures