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/snapshot.test.ts

@@ -4,7 +4,8 @@
  * These are the surface the GUI consumes across Electron IPC. `snapshot()` is
  * synchronous — it reads the in-RAM store and projects it into plain records
  * (no keys) — and `subscribe({ onChange })` delivers a `LibraryChange` whenever
- * a background refresh actually changes the derived records. The contracts:
+ * a refresh or `refreshCollection()` changes the derived records. The
+ * contracts:
  *
  *  1. `snapshot()` deduplicates a file across memberships into one record with
  *     every album id, orders photos newest first, and carries no key material.