In times of uncertainty, there’s comfort in constancy. The always-there markers that remind us of a gentler, more refined era. In 21st-century Manhattan, those markers include a roster of usual suspects: Central Park, the Carlyle Hotel, Bryant Square, the elegant art deco towers and discreet co-op apartments of the Upper East Side. There’s a reassuring solidness attached to many of these early 20th-century constructions, so well preserved by care-taking residents over more than a century.