Shipped 2026-09-28: MasterPages.PathFor reads the site's MasterPageUrl from the property bag (Site Settings > Theme writes it) and resolves ~masterpage/x.master to wwwroot/XSLT/x.xslt, falling back to MasterPage.xslt when the file does not exist; wired into PagesController (both render paths) and BoardChromeShell. Closes risk-7. The gallery today has one master page; MasterPageEngine stays as is (unused; delete when the Theme page lists the gallery).