Fix data race possible from safe code on ClusterPointInner

[Ollie-Pearce]

The ClusterPointInner type implements Send and Sync, allowing for instances to be shared across threads. The type contains a Cell field and methods on the type make unsynchronised accesses to this field.

#[derive(Debug, Clone)]
pub struct ClusterPointInner<Id> {
    pub id: Id,
    pub cluster: Cell<Option<usize>>
}

unsafe impl<Id> Send for ClusterPointInner<Id> {}
unsafe impl<Id> Sync for ClusterPointInner<Id> {}

Reproduction:

Test case:

#[test]
fn clusterpointinner_race() {
    let point: ClusterPointInner<bool> = ClusterPointInner::new(true);
    std::thread::scope(|s| {
        s.spawn(|| {
            let _ = point.clone();
        });
        point.assign_cluster(1);
    });
}

Verifying with miri:

cargo +nightly-2025-08-20 miri test -p gaoya --test repro

Output:

test clusterpointinner_race ... error: Undefined Behavior: Data race detected between (1) non-atomic write on thread `clusterpointinn` and (2) non-atomic read on thread `unnamed-2` at alloc40184
    |
546 |         unsafe { *self.value.get() }
    |                  ^^^^^^^^^^^^^^^^^ (2) just happened here
    |
help: and (1) occurred earlier here
    |
 31 |         self.cluster.set(Some(cluster_id));
    |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

Fix:

• Remove Send and Sync implementations from ClusterPointInner in clustering_serial.rs
• If I understand correctly while the ClusterPointInner type in clustering_parallel.rs does need to be Send and Sync the trait implementations are not required for the equivalent type in clustering_serial.rs