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 before it listed one.
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, and saves it. It runs in the same slot as
a refresh, so the two never run at the same time, and `close()` waits
for it. It never stores a collection that a refresh has already dropped.
It leaves the refresh cursor alone. `open()` therefore awaits the first
refresh on a cache whose cursor is still 0. `quak files` with no
argument still waits for a refresh.

Model: opus-5-5

src/library/index.ts

@@ -35,7 +35,7 @@
 // A refresh stages all of its network work first and only mutates the store
 // once every fetch has succeeded. A refresh that fails partway therefore never
 // becomes visible to reads: the last good snapshot stays in place, and the
-// failure surfaces through `onProgress` and `status()` instead. A refresh that
+// failure surfaces through `onProgress` and `status()` instead. A commit that
 // mutates RAM but then fails to persist keeps `status().lastError` set and the
 // store marked unsaved until a later save actually lands, so a stuck disk is
 // never masked by a subsequent empty refresh.