Lee Chang-dong movies abound in stellar performances — see also Yoon Jeong-hee in “Poetry” and Sol Kyung-gu and Moon So-ri in “Oasis” — but none is as devastating as Jeon Do-yeon is in “Secret Sunshine.” Her tear-streaked turn is grief embodied, and a reminder that the kind of tragedies you imagine only befalling other people can and will eventually befall you.