Upsert glossary terms

Creates or updates 1–100 terms ({ entries: [...] }; send one term as a one-item array), matched by keyword, in one atomic batch. An absent field keeps its stored value. null clears a nullable field or removes a map entry; other fields reject null. In batches and imports, items match stored items by natural key, stored items absent from the batch are untouched, and nothing else is deleted. A blank CSV cell counts as absent. Repeating a keyword within one batch rejects the whole batch (400). Requires org:context:write.