docs(compact): correct pruning surface contracts

docs/cordis-catalog/services.md

@@ -1147,6 +1147,8 @@ pruneContent(blocks: readonly ContentBlock[]): ContentBlock[] | null
  * and points at the shadowed node for durable provenance and replay.
  * @param session - session whose current surface is rewritten.
  * @returns landed replacements and aggregate Unicode-code-point savings.
+ * @throws when the session rejects a replacement; replacements committed
+ * earlier in the pass remain durable.
  */
 pruneSession(session: Session): PruneResult
 ```