First real Windows build surfaced errors that a Linux host couldn't catch:
* Enable the missing `windows` feature gates (Win32_System_LibraryLoader,
Win32_System_SystemServices, Win32_Security — the last brings in the
`Ex` registry APIs) so the imports resolve.
* Depend on `windows-core`: the `#[implement]` macro expands to
`windows_core::` paths, which need the crate nameable in the root.
* `IClassFactory::LockServer` takes `Win32::Foundation::BOOL`, not a
`core` BOOL.
* Name the returned image type via a new `dwg_thumbnailer::RgbaImage`
re-export instead of a direct `image` dependency.
Verified with `cargo check --target x86_64-pc-windows-gnu -p
dwg-thumbnailer-win` (and the native check still passes).
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
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