ML data that does not decrypt is recorded with its reason (closes #198)

`fetchMLDataBatch` dropped an entry that did not decrypt, gunzip or parse,
so the file looked like one with no ML data and the run exited 0. It now
returns the reason beside the payloads, also for ML data that is not a
JSON object. The library gives the reasons from its last ML data fetch
that succeeded in `status().mlDataErrors` and the `fetchMLData` "done"
event; a fetch that fails leaves them as they were. `quak backup` writes
the reason as `mlDataError` and records the file in `failures.json`;
`backup-metadata` writes `mlDataError` and exits 1.

Judgement call: a file whose ML data is cached keeps it in its JSON when a
newer copy does not decode, as when the fetch fails.

Model: opus-5-5

TODO.md

@@ -17,16 +17,28 @@ pre-1.0
 
 # Next Step
 
-Two bugs are open: https://git.eeqj.de/sneak/quak/issues/198 and
-https://git.eeqj.de/sneak/quak/issues/205. 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.
+One bug is open: https://git.eeqj.de/sneak/quak/issues/205. 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
 
+- 2026-10-16: ML data the server sends that does not decrypt, gunzip or parse is
+  no longer taken for no ML data (issue 198). `fetchMLDataBatch` returns why for
+  each such file beside the payloads; ML data that parses to something other
+  than a JSON object, such as `null`, is one of them. The library keeps the
+  reasons from its last ML data fetch that succeeded in memory, not in the
+  cache, and gives them in `status().mlDataErrors` and in the `fetchMLData`
+  event with status `"done"`; a fetch still running or one that fails does not
+  change them. `quak backup` writes the reason as `mlDataError` in the file's
+  JSON when no ML data is cached for it, and records the file in `failures.json`
+  with an error starting `ML data: `; `backup-metadata` writes `mlDataError`
+  too. Both exit 1. Tests serve ML data encrypted under a key that is not the
+  file's.
+
 - 2026-10-16: With `--verify`, an original whose hash does not match is no
   longer removed before its replacement is downloaded (issue 199).
   `replaceOriginal` in `src/library/content.ts` downloads the replacement to a