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. It leaves the refresh
cursor alone, so the next refresh still fetches every other collection
that changed. `refreshOnce` and `refreshCollection` share the commit
step. `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