Steven Spielberg is Hollywood’s perennial Pollyanna. Through half a century of creature features, epic disasters, sprawling adventures, quirky character studies and electrifying political thrillers, Spielberg has maintained his hope. He’s also let his hand get heavier, painting his optimism in broad, unmistakable strokes for anyone searching for something to hold onto in an increasingly divided and combative world outside the movie theater. But his highly anticipated return to the sci-fi summer blockbuster, the brilliant and beguiling “Disclosure Day,” is Spielberg’s most shameless appeal toward idealism yet. So unabashed, in fact, that his earnestness makes a familiar allegory take on powerful new resonance, as if Spielberg were trying to convey that humanity can still have a do-over. Desperately trying to retrieve a trove of stolen government secrets that will prove the existence of extraterrestrial life, Noah Scanlon (Colin Firth) — the director of a secret military contractor, Wardex — warns of the ramifications to a Wardex defector, Hugo (Colman Domingo). “It is a virus for which humanity has no cure,” Noah tells his former colleague, implying …