addon.pathtools.cornerSet and cornerRemove now refuse a key other than
id and anchors in an `items` member, with the standard unknown-parameter
error, instead of ignoring it. A misspelled `anchor` used to fall back
to the request's `anchors`, else the path's direct-selected anchors,
else every corner of the path. They also refuse `items` given together
with `ids`, which `items` replaces. Saved steps played with replay
still run as recorded.
Docs: docs/mcp.md, docs/target-app-parity.md.