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` for a file with no ML data cached and records
the file in `failures.json`. `backup-metadata` writes `mlDataError` for it
and exits 1; `runMetadataBackup` returns `filesWithMLDataError`.

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

@@ -17,8 +17,7 @@ pre-1.0
 
 # Next Step
 
-Three bugs are open: https://git.eeqj.de/sneak/quak/issues/198,
-https://git.eeqj.de/sneak/quak/issues/199 and
+Two bugs are open: https://git.eeqj.de/sneak/quak/issues/199 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.
@@ -28,6 +27,19 @@ 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: The `~/bin/quak` that `make install` builds retries a refused
   connection and a name that does not resolve, and the backup records either as
   transient (issue 206). Bun's `fetch` reports them with codes of its own,