fix: `grind` cast internalization order

This PR fixes a `grind` internal error triggered when `cast` (or `Eq.rec`,
`Eq.ndrec`, `Eq.recOn`) is applied to an argument that has not yet been
internalized. `pushCastHEqs` was emitting `e ≍ a` before internalizing
the args of `e`, so the `rhs` of the heq had no enode and the debug
sanity check tripped. The call now runs after the args are internalized.