Load modes, and why listings need preloading

• PreloadNone (.collection(name)): don’t preload anything, just fill the cache while working;
• PreloadAllDocs (.collection_with(name, |c| c.preload_all())): preload all documents in the collection;
• PreloadAllIfEmpty (.collection_with(name, |c| c.preload_all_if_empty())): preload all documents unless an earlier run finished downloading the collection. This is useful for the persistent cache, where the saving applies when the cache loads; for the memory cache it is the same as PreloadAllDocs, since an in-memory cache always starts empty. The persistent cache keeps a marker for each collection whose download has finished, and skips the download only for a collection with a marker. A collection whose download was interrupted, for example because the process was killed, has no marker: its rows are discarded and it is downloaded again on the next start. invalidate_all and a resync after Firestore resets the listener target remove the marker too, and each costs one more download on the next start.

.preloaded_collection(name) picks the appropriate preloading mode for the backend.

A lazily filled collection holds only the documents that happened to be read through it. Answering a list or query from it would return a subset that looks like a complete answer, so the library refuses to do so: read_through_cache quietly falls back to Firestore, and read_cached_only returns an error naming the collection. If partial results are genuinely acceptable, opt in with .incomplete_collection_policy(FirestoreCacheIncompleteCollectionPolicy::PartialResults).