Discard a cache whose list of what did not decrypt is damaged

metadata.json's list of the albums and files the refresh could not
decrypt is read entry by entry, so a value that is not a list of records
discards the cache as other damage does, instead of failing every later
backup. "Backup layout" now says that an album, or a file's copy in an
album, that quak can no longer decrypt loses its folder, JSON or link as
a deleted one does, and the originals stay. It no longer mentions the
empty title in failures.json, which no test covers.

Model: opus-5-5

README.md

@@ -746,14 +746,14 @@ library's `lib.backup({ includeThumbnails: true })` also writes
 `thumbnails/<fileID>.jpg` beside `collections/`; `quak backup` does not.
 
 A file in the backup's scope that the library could not decrypt (see "Default
-reads vs. fresh reads") goes into `failures.json` with an empty title, since its
-title is encrypted too, and an error that starts `could not be decrypted:` and
-gives the reason. If a copy of it in another album decrypted, it does not: its
-original is stored from that copy, and the album where it does not decrypt
-neither links nor lists it. An album the library could not decrypt has no name
-quak can read and no files it can list. A run without `--json` prints one line
-for it, `FAILED album <id>: could not be decrypted: <reason>`, and `--json` and
-the `BackupResult` list its ID in `undecryptableAlbums`. Such a file in
+reads vs. fresh reads") goes into `failures.json` with an error that starts
+`could not be decrypted:` and gives the reason. If a copy of it in another album
+decrypted, it does not: its original is stored from that copy, and the album
+where it does not decrypt neither links nor lists it. An album the library could
+not decrypt has no name quak can read and no files it can list. A run without
+`--json` prints one line for it,
+`FAILED album <id>: could not be decrypted: <reason>`, and `--json` and the
+`BackupResult` list its ID in `undecryptableAlbums`. Such a file in
 `failures.json`, or such an album, keeps the exit code at 1, run after run,
 until the server sends a copy that decrypts or deletes it; the next run then
 drops it.
@@ -794,9 +794,12 @@ both, each named after the title with that file's extension (`IMG_0001.heic` and
 
 Each run removes the symlinks into the date folders that no longer belong in
 their collection's directory, and the directories (and JSON) of collections that
-were deleted or renamed. Nothing else in `collections/` is touched: a file or a
-symlink you put there stays, and a directory that still holds one after its
-symlinks are removed stays too, with its JSON.
+were deleted or renamed. An album quak can no longer decrypt loses its directory
+and JSON the same way, and a file whose copy in an album quak can no longer
+decrypt loses its symlink there and its entry in that album's JSON; the
+originals stay at their save paths. Nothing else in `collections/` is touched: a
+file or a symlink you put there stays, and a directory that still holds one
+after its symlinks are removed stays too, with its JSON.
 
 Each file is downloaded exactly once regardless of how many collections it
 appears in, and written once: straight to its save path, with no copy left in