DirStructureAsync: carry the struct's where clause over into the generated future enums

[KiSetsuFu-PuLiN]

#[derive(DirStructureAsync)] inherits the derived struct's where clause in the generated trait impls, but not in the generated type Future enums — those are emitted with the per-field bounds only. When a struct's own where clause carries bounds that the generated code needs (e.g. bounds required by a custom Filter for a generic path type), compilation fails inside the derive expansion even though the bounds are written on the struct itself.

• KiSetsuFu-PuLiN/dir-structure@2905a34 — compile-time regression test (derive_where_clause module): without the fix it fails with <Vfs as VfsCore>::Path: MyTrait is not satisfied.
• KiSetsuFu-PuLiN/dir-structure@cf1f4f9 — merge the struct's where clause into the where clause of the generated future enums (future_impl_enum in dir_structure_async/{read_from_async.rs, write_to_async_ref.rs}), consistently with what the impl generation already does.