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

README.md

@@ -685,17 +685,20 @@ library. The read commands — `collections`, `files`, `get`, `get-thumb`,
 `helper fix-missing-thumbnails` — force a fresh server round-trip before they
 answer, so they report current account state rather than whatever the cache last
 held. `collections` asks the server for the collection list alone, in one
-request, and neither fetches any collection's files nor changes the cache. The
-others wait for a refresh. With an empty cache, as on a first run, that refresh
-fetches the files of every collection, one collection after another, and stores
-nothing until it has them all, which on a large account takes a while. Meanwhile
-the command logs a line on stderr for each collection whose files it has
-fetched, such as `Fetched the files of 3 of 120 collections`, with `--json` too;
-`backup` does the same while the refresh it starts with runs. stdout holds only
-the command's output. If that round-trip fails, the command prints the error on
-one line and exits 1, or 3 when the server no longer accepts the saved session
-(see "Session handling"). An album or file quak cannot decrypt does not fail it;
-the command answers from what did decrypt (see "Default reads vs. fresh reads").
+request, and neither fetches any collection's files nor changes the cache.
+`files` with a collection ID asks for the collection list the same way, then for
+the files of that collection alone, and stores them in the cache. It asks for
+none of them when the cache's copy of that collection is current. The others
+wait for a refresh. With an empty cache, as on a first run, that refresh fetches
+the files of every collection, one collection after another, and stores nothing
+until it has them all, which on a large account takes a while. Meanwhile the
+command logs a line on stderr for each collection whose files it has fetched,
+such as `Fetched the files of 3 of 120 collections`, with `--json` too; `backup`
+does the same while the refresh it starts with runs. stdout holds only the
+command's output. If that round-trip fails, the command prints the error on one
+line and exits 1, or 3 when the server no longer accepts the saved session (see
+"Session handling"). An album or file quak cannot decrypt does not fail it; the
+command answers from what did decrypt (see "Default reads vs. fresh reads").
 `--cache-dir` overrides where the cache lives; without it each account gets its
 own directory under the per-user cache path.
 
@@ -1180,6 +1183,15 @@ since the stored copy, so a collection stored without its files would never get
 them. `quak collections` uses it (issue
 https://git.eeqj.de/sneak/quak/issues/210).
 
+`await lib.refreshCollection(collection)` brings one collection up to date in
+the cache, given its `Collection` from `freshCollections()`. It does for that
+collection what a refresh does for a changed one, and fetches no other
+collection's files. It sends no request when the cache's copy has the same
+`updationTime`. Otherwise it fetches the collection's files changed since that
+copy, or all of them when there is none, then stores and saves them. The refresh
+cursor stays where it was, so the next refresh still fetches the files of every
+other collection that changed. `quak files` with a collection ID uses it.
+
 An album or file in the server's answer that quak cannot decrypt, or whose
 metadata is not JSON, does not fail the refresh. The refresh leaves it out, an
 album with all its files, and drops any older copy the cache held, so reads