copyByJSONTag previously skipped any IsZero value, which made it
impossible for tasks_update / projects_update to flip done from true
to false, reset priority/percent_done to 0, or unarchive a project.
A non-nil pointer src is now the unambiguous "caller supplied this"
signal: dereferenced values are written through even when zero, while
value-typed src fields keep the partial-update semantics. The
affected wrapper fields (Done, IsArchived, IsFavorite, Priority,
PercentDone, RepeatAfter, RepeatMode, BucketID,
CoverImageAttachmentID, ParentProjectID, Position) move to pointer
types so the JSON Schema still marks them optional.
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