By injecting solid objects overnight as well as heavy drilling fluid into the stricken well leaking oil into the Gulf of Mexico, engineers appeared to have stemmed the flow of oil, Adm. Thad W. Allen of the Coast Guard, the leader of the government effort, said on Friday morning. But he stressed that the next 12 to 18 hours would be “very critical” in permanently stanching what is already the worst oil spill in United States history.