## Summary

Replace GPUI's shared asset futures with owning cache entries. The
mutex-backed waiter collection in futures::future::Shared can contend
between a worker and the browser main thread, where blocking waits are
forbidden.

- Keep loading on the background executor, with one foreground task
publishing the cached result and notifying interested views.
- Deduplicate loads and view registrations; cancel pending work on
eviction without allowing completion to update a replacement entry.
- Change App::fetch_asset to return Option<A::Output>, remove
ImageLoadingTask, and make ImageCacheItem an opaque owning entry.
- Migrate the retain-all image cache and image gallery LRU example.
Repository-wide search found no further callers requiring migration.

Results become visible after the foreground completion task publishes
them. Entries expose neither cloneable nor awaitable task handles.

Release Notes:

- N/A