The cases are the incidents: a worker failure must leave the stored union alone
instead of overwriting it with "null" (50ca9cc), an incremental merge must refuse
to run on state that is not exactly one behind, recomputes must coalesce while
adds must not be dropped, and legacy telegram subscriptions (no radius, no
location) must be skipped without throwing.
Also the 16 MiB case the FIXME asked for: a 20 MB union comes back under the cap,
still parseable, with the degradation recorded.
