In leader mode, the layered cursor performs a full layered-level lookup before insert/update with overwrite=false. This is redundant: leaders have no ingest constituent, all data is in the stable btree, and the btree cursor already enforces overwrite semantics (stable_cursor->update returns WT_NOTFOUND for missing keys; stable_cursor->insert returns WT_DUPLICATE_KEY for existing keys). The lookup should be skipped for leaders and the btree-level error returns used directly instead.