refactor(loader): deduplicate negotiation types via generated bindings

[MungbeanOuO]

Fixes #191

(Note: The second issue mentioned in #191 regarding LoaderInterfaceStructs naming was previously resolved in #204. This PR addresses the remaining duplication issue.)

Changes

• Deduplication: Removed all hardcoded negotiation structs and enums (e.g., XrNegotiateLoaderInfo, LoaderInterfaceStructureType) from the loader module.
• Integration: Updated the module to rely entirely on openxr_sys generated types.
• Metadata Restoration: Added manual impl blocks to the generated types to provide the necessary TYPE and VERSION constants (which are missing from the XML registry).

Breaking Change

This removes the manually defined types in openxr_sys::loader. Downstream users must migrate to the generated types in the crate root (e.g., XrNegotiateLoaderInfo -> NegotiateLoaderInfo).

[Ralith]

Thanks! It looks like this module no longer exports any top-level items -- should we make the module itself private now?

[MungbeanOuO]

I agree. Since there are no exported types left in this module, hiding it from the public API surface seems appropriate. I'll make it private.