This PR fixes universe unification for `for` loops with `mut` variables
whose types span multiple implicit universes. The old approach used
`ensureHasType (mkSort mi.u.succ)` per variable, which generated
constraints like `max (?u+1) (?v+1) =?= ?u+1` that the universe solver
cannot decompose. The new approach uses `getDecLevel`/`isLevelDefEq` on
the decremented level, producing `max ?u ?v =?= ?u` which `solveSelfMax`
handles directly.
Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>
