Every dialect projects IS_NULLABLE and nothing read it. The browse session now reads
it (tolerantly: a dialect that does not project it reads as unknown) and the picker's
descriptor line says 'nullable' / 'NOT NULL', so an operator knows a cell can come
back NULL (Uncertain, or Bad under nullIsBad) before committing the tag. The runtime
catalog loader keeps reading names only - nothing at runtime consumes nullability.
Refs #744
Claude-Session: https://claude.ai/code/session_01Ub9hGmYLQvn9XETzw7CMsT
Joseph Doherty
7f5d959fb9