quak backup works on up to the content pool's limit of originals at once (closes #182)

lib.backup() ran its originals one at a time, so the content pool's five
slots, or the contentConcurrency of the pools a library was opened with,
went unused. The work for one original, its verify hashing included, is
now storeOriginal, and that many workers take files from one iterator
over the backup's files, so no more originals than the limit are open at
once. BackupLibrary gains contentConcurrency, which the library sets from
its content pool. The README's "Request pools" says so. The tests of a
backup whose lock is lost now expect all three originals downloaded,
since all three are under way at once.

Model: opus-5-5

TODO.md

@@ -17,18 +17,23 @@ pre-1.0
 
 # Next Step
 
-`quak backup` downloading one original at a time,
-https://git.eeqj.de/sneak/quak/issues/182, is fixed in
-https://git.eeqj.de/sneak/quak/pulls/192, which waits on review. No other
-implementation work is open. The check of quak against a live Ente account,
-https://git.eeqj.de/sneak/quak/issues/187, needs an account only sneak has, so
-it waits on him.
+None: no implementation work is open. The check of quak against a live Ente
+account, https://git.eeqj.de/sneak/quak/issues/187, needs an account only sneak
+has, so it waits on him.
 
 Tagging and releases are decided by sneak alone, and happen only when he
 declares one.
 
 # Completed Steps
 
+- 2026-10-14: `lib.backup()`, and so `quak backup`, works on up to the content
+  pool's limit of originals at once, 5 by default, instead of one at a time
+  (issue 182). That many workers take files from one shared list, each doing all
+  the work for one original, its `--verify` hashing included, so no more
+  originals than that are open at once. `BackupLibrary` says the limit as
+  `contentConcurrency`. A test holds each download at the fake Ente server to
+  count those in flight.
+
 - 2026-10-14: Next Step said the cache design, issue 36, waits on sneak's
   review, though that issue closed on 2026-10-03 (issue 194). It now names the
   issues open on the tracker.