A radius exported as `--radius: 12` was invalid where CSS reads it as a
length (`border-radius: var(--radius)` ignores a bare number). Number
values now carry px unless the layers bound to them, their scopes, or the
variables aliasing them make them an opacity or a font weight, which stay
bare. The roles come from a small token_roles module.
Updates the variables_io_tests expectation for the radius (4 → 4px,
6.5 → 6.5px), and adds a test for a bound radius, a bound opacity and an
opacity alias's target.