quak files COLLECTION fetches that collection's files alone (closes #217)

`quak files COLLECTION` used to wait for a full refresh, which on an
empty cache fetched the files of every collection first. It now reads
the collection list, then calls the new `Library.refreshCollection()`.
That brings one collection up to date in the store as a refresh would.
Once the stored cursor is past 0, it only updates a collection the store
holds. For a new one, `quak files COLLECTION` waits for a refresh. A
refresh from a cursor of 0 replaces all the store holds, so collections
then sit in the server's order. `open()` awaits the first refresh while
the cursor is 0. The store lists what did not decrypt in a fixed order.

Model: opus-5-5

test/library/library.test.ts

@@ -20,9 +20,9 @@
  * 5. `close()` stops the timer and is idempotent, and its promise resolves
  *    only once an in-flight refresh has written the cache file.
  * 6. `cacheDirectory` defaults to the env-paths cache dir plus the user id.
- * 7. `open()` branches on the cache: an empty cache awaits the first refresh
- *    (it has nothing to serve yet); an existing cache serves its copy at once
- *    and refreshes in the background, so a slow or dead server never stalls
+ * 7. `open()` branches on the stored cursor. At 0, as on an empty cache, it
+ *    awaits the first refresh. Past 0 it serves the cache's copy at once and
+ *    refreshes in the background, so a slow or dead server never stalls
  *    opening.
  * 8. A save failure that leaves RAM ahead of disk keeps `status().lastError`
  *    set and keeps retrying the write; a later empty refresh does not clear it.