`casement` is ZSeven-W's winit fork (sibling repo, referenced by
local path for now). It adds the macOS open-documents Apple-event
hook that upstream winit lacks, needed for Finder double-click open.
The `package = "casement"` key keeps the `winit` import name so all
`use winit::…` stays unchanged.
glutin-winit is dropped: it hard-depends on the upstream `winit`
package, which would pull a second, incompatible winit into the
tree. Its only use — GlWindow::build_surface_attributes — is replaced
by a direct glutin SurfaceAttributesBuilder call in provider.rs.
0730230ddb