WASHINGTON — At 79, Anthony Fauci has become the grandfatherly captain of the coronavirus crisis.Through unrelenting appearances both in the media and onstage with the president and his lieutenants, the longtime head of the National Institute of Allergy and Infectious Diseases has been a reliable constant in a time of uncertainty.As recently as a few weeks ago, it seemed as if the gravest threat facing the country was the fact that reality had split along partisan lines, creating unresolvable disagreements about what was happening in America and why.Now a public-health catastrophe has remade our reality and pushed Fauci into the spotlight as a figure that might have seemed impossible less than a month ago: a government expert with an unwelcome message who is nonetheless regarded as a truth-teller by the president, Democratic leaders and media figures alike.