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()`, which brings that one collection up to
date in the store as a refresh would, and saves it. It sends no request
for the files when the stored copy is current, and it keeps a newer copy
that a refresh stores while it fetches. It leaves the refresh cursor
alone, so the next refresh still fetches every other collection that
changed. `quak files` with no argument still waits for a refresh.

Model: opus-5-5

TODO.md

@@ -26,6 +26,15 @@ declares one.
 
 # Completed Steps
 
+- 2026-10-25: `quak files COLLECTION` fetches the files of that collection alone
+  (https://git.eeqj.de/sneak/quak/issues/217). It used to wait for a refresh,
+  which on an empty cache fetched the files of every collection first. It now
+  reads the collection list through `lib.freshCollections()`, then brings that
+  one collection up to date through the new `lib.refreshCollection()`. That
+  stores and saves its files as a refresh would, and leaves the refresh cursor
+  where it was. When the stored copy is current, it asks for none of the files.
+  `quak files` with no argument still waits for a refresh.
+
 - 2026-10-22: The JSON `quak backup` and `quak backup-metadata` write for a file
   holds `keywords` in place of the CLIP embedding in its ML data
   (https://git.eeqj.de/sneak/quak/issues/218). The keywords are the titles of