Opt-in warning for `UNION`s with only one piece

A new `-Weverything`-level warning (e.g. `-Wsingle-union` or `-Wone-block-union`) that detects `UNION`s without any `NEXTU` blocks, which may be a mistake or redundant. (I'm requesting this after having real-world cases where it would be useful.)

Start by locating the assembler handling for UNION and NEXTU, then inspect how existing -Weverything-level warnings are registered and tested. Define the warning's behavior for UNIONs with no NEXTU blocks and confirm completion with focused diagnostic tests.

Written by the indexing model from the issue text.