Use the public transform-free subset API on Paradox 2

[mb706]

Paradox 2 deliberately makes detached Domain internals non-authoritative and
provides ParamSet$subset(..., keep_trafo = FALSE) for constructing a
transformation-free acquisition domain. Use that public API on Paradox 2
instead of deleting the private .trafo column returned by $domains.
Retain both historical implementations for Paradox 1, so the package remains
dual-version compatible. Extend the existing acquisition-domain regression to
verify that producing the detached domain does not remove transformations
from the source search space, and record the migration in NEWS.

• Use public transform-free Paradox 2 subsets
• Document the Paradox 2 subset migration