The "This event only" edit path built nested JMAP patch pointers
(recurrenceOverrides/<recurrenceId>/<field>). On the first edit of an
occurrence the override object doesn't exist yet, and JMAP pointers can't
create a missing intermediate object, so Stalwart returned notUpdated and
the edit failed with two error toasts.
Build the override as a single pointer recurrenceOverrides/<recurrenceId>
whose value is the occurrence's field set (dropping identity/series keys),
mirroring the working "delete this occurrence" path. Extracted as the pure,
tested helper buildRecurrenceOverridePatch.
Closes#774.
Linus Rath
Stefan Hildebrandt