Check the lock before every file, and pass the lock itself to the backup

The backup now checks its lock after the refresh and before each file of
the thumbnail, JSON and album passes, as well as before each original and
before failures.json. lockBackupDirectory returns the lock, with release()
and lost, in place of a release function, and lib.backup() takes it as
lock in place of lockHeld: true; quak backup passes it. This drops the
process-wide map from directory path to lock, which missed a lock taken
through another path to the same directory.

Model: opus-5-5

TODO.md

@@ -27,12 +27,13 @@ declares one.
 
 - 2026-10-14: A backup whose lock is lost no longer dies with a stack trace
   (issue 184). `lockBackupDirectory` gives proper-lockfile an `onCompromised`
-  that records why the lock was lost instead of throwing from its timer. The
-  backup then stops before it starts another original, and before it writes
-  `failures.json`, with `lost the lock on DIR: REASON`, which `quak backup`
-  prints as one line before it exits 1. Releasing a lost lock leaves
-  `backup.lock` alone. Tests in `test/cli/backup-end-to-end.test.ts` remove the
-  lock, or put another backup's in its place, while a download is held open.
+  that records why the lock was lost instead of throwing from its timer, and
+  returns the lock, which `quak backup` passes to `lib.backup()` as `lock`. The
+  backup then stops before it starts another file with
+  `lost the lock on DIR: REASON`, which `quak backup` prints as one line before
+  it exits 1. Releasing a lost lock leaves `backup.lock` alone. Tests in
+  `test/cli/backup-end-to-end.test.ts` remove the lock, or put another backup's
+  in its place, while a download is held open.
 
 - 2026-10-14: `quak backup` no longer asks the server for changes every 3
   seconds while it runs (issue 183). It opens its library with