RHDH Backstage API Changes

A single home for the NFS API deltas. Two skills need this material and neither owns it, so it lives here and both invoke it by name.

Who invokes this

• /rhdh-backstage-upgrade owns version numbers. It arrives here when a bump moves a checkout across one of these deltas and NFS code stops compiling.
• /rhdh-plugin-nfs-migration owns extension shape. It arrives here when a plugin was already migrated against an older alpha, or when a Blueprint param from an older guide produces a TypeScript error.

That is also the boundary between those two skills. A version number is the upgrade skill's business; which Blueprint an extension becomes and where it attaches is the migration skill's. This skill sits under both and belongs to neither: it says only what changed in the API, not which change you should make.