Fix SortedSet.new(sorted_set) on Ruby 4.0

[joshuay03]

On Ruby 4.0, SortedSet.new(another_sorted_set) always returns an empty set. The Ruby 4.0 implementation overrides replace to handle a SortedSet argument by copying @tree directly, but C's Set#initialize does not call the Ruby level replace when given a Set subclass. It
takes a fast path that bypasses our overrides, so @tree is never populated and the new instance behaves as if empty.

The fix mirrors the existing replace logic inside initialize: if the argument is already a SortedSet, copy its @tree directly and pass nil to super so the C initializer sets up an empty base without attempting to populate it. All other arguments fall through to super as before, preserving block transformation and all existing behaviour.

I would appreciate a release with this fix if possible. We are in the middle of a Ruby 4.0 upgrade and this is a blocker for us.

cc @knu

[joshuay03]

Bump 🙏🏽

[knu]

This makes sense to me.