fix: keep `Nat.reduceSubDiff` off where `Init.Data.Nat.Simproc` is not imported

The `pn = nn` cases of `Nat.reduceSubDiff` used `Nat.add_sub_cancel` and `Nat.add_sub_add_right`, which are available in modules that do not import `Init.Data.Nat.Simproc`. Since a simproc case fires only if its lemma is in the environment, the simproc started firing in those stdlib modules, breaking `List.set_append` once stage0 is updated. This commit uses copies in `Nat.Simproc`, restoring the previous behavior.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>