Implements the wlr foreign-toplevel management protocol on top of the
existing ToplevelInfoState/ToplevelManagementHandler, so waybar's wlr/taskbar
module works. cosmic-comp previously shipped only ext-foreign-toplevel-list,
which enumerates but cannot activate or close.
The wlr manager state is constructed alongside ForeignToplevelListState and
driven from the same new_toplevel/remove_toplevel/refresh hooks, so no
separate registration in state.rs is needed.
Accessors global_id and registered_toplevels moved to an unbounded impl
block: adding the wlr dispatch bounds to the shared impl made them viral and
broke overlap_notify.rs, which needs neither.
cargo check passes clean. NOT runtime-verified — no Wayland session here.
