Offer to switch to another installed version when a project plugin is unavailable

When a solution's exact plugin package is unavailable, Studio reports what is missing (id, version, required archive hash) but offers no next action even when other versions of the same plugin are installed in the Plugin Library.

Suggested behaviour: when another version of the same package id is installed, the missing-package surfaces (the load notice, the Navigator label's details, the Details dialog, and the build-block message) should list the installed versions and offer to switch the project or shared reference to one of them. Switching is an explicit project change: it runs the same validation as adding a plugin, so if the new version renames or removes members the project uses, Studio lists the compile errors and offers Add Anyway / Update Anyway, and the change stays undoable.