Obligation descriptions render a Unit-literal argument as <expr>: mk(()) reads mk(<expr>) in verify --json and E505 text

Found during the #1213 burndown (PR C2). `vera/ast.py`'s `format_expr` has no UnitLit arm, so an obligation over `mk(())` describes the call as `mk(<expr>)` in `verify --json` descriptions and E505 message text — two Unit-argument calls are indistinguishable by description (spans still differ). One rendering arm; small. Lands in the burndown's hygiene tail (C.6) or the morning note carries it.