The `is_sync::<T>()` runtime check relied on implicit specialization via
`Copy`/`Clone` array behavior, which has changed in Rust 1.86+.
`UserDataRef` always taking an exclusive lock even for `Sync` userdata,
preventing concurrent shared borrows.
With the `send` feature flag enabled, userdata types must now be `Send + Sync`.
This is a breaking change, `T: Send + !Sync` userdata types can be wrapped in a `Mutex`
or used inside a `Scope` where this restriction is lifted.
Alex Orlenko