Harden ListViewModel fetch synchronization

- Fixes _vmCache concurrency with copy-on-write cache publication.
- Preserves latest-fetch-wins behavior across overlapping RPC GetItems() calls.
- Prevents stale or canceled fetches from publishing and makes them abort promptly.
- Improves cancellation cleanup for abandoned item view models and replaced token sources.
- Updates empty-state tracking to follow overlapping fetch activity correctly.
- Reduces hot-path cache overhead by removing per-item cache locking and full cache rebuilds.