fix: Harden FDv1 data source destination and status-manager lifetimes

[beekld]

Summary

The FDv1 streaming and polling sources received their destination and status manager as a raw pointer and reference. A source callback still in flight during teardown could reach those objects after the owner freed them. This is the use-after-free reported in #602.

The handoff now uses smart-pointer handles.

• IDataSynchronizer::StartAsync takes a shared_ptr<IDestination>.
• The sources observe the destination through a weak_ptr and hold the status manager as a shared_ptr. Neither can dangle.
• The FDv2 fallback adapter, BackgroundSync, and ClientImpl own these objects via shared_ptr.

The concrete sources already guarded their own callbacks, so the reported crash was latent rather than reachable through the public API. This change makes the lifetime safe by construction regardless of source behavior.

No public API change. ClientImpl::DataSourceStatus() returns the same reference as before.