Maggie O’Farrell first published “Hamnet” in March 2020 — a timing which some consider ill-fated, especially given the novel’s theme of familial grief. While the film and the novel take creative liberties, Hamnet — the son of Shakespeare — died in childhood. Mescal’s portrayal, especially in the first half of the film, feels magnetic, especially in regard to his chemistry with Buckley. Let it be known: “Hamnet” isn’t a film you leave as soon as it ends. Long after the screen fades to black, “Hamnet” remains, tender yet haunting, much like the memory of a child gone too soon.