Colson Whitehead essay about when he first defined himself as New Yorker through memories of riding subway; says New Yorkers always see city through eyes of first glimpse; each person defines it for him or herself from personal experience, and everyone's experience of city is unique; small changes go unheralded and neighborhoods morph from one thing to another in almost-unnoticed steps as familiar shops become chain stores; compares streets to calendar, marking time and progress of each individual; mourns World Trade Center collapse, but claims towers are still there, that each New Yorker carries them in memory; photo (M)