Rivet 0.6.1

• Fix the 0.6.0 family-release path: manifest-compose --output no longer
  treats the destination as another input, every release leg's Ed25519
  signature and key ID are verified before aggregation, and the command is
  listed in raco rivet help.
• Make portable update archives honor the documented
  <name>-<version>-<os>-<arch>.zip convention and point per-platform update
  manifests at those archives instead of the separately published installer.
• Carry linux-binary-name through deb/rpm desktop entries, AppImage staging
  and AppRun, dependency collection, and production verification. Native
  Linux formats now work with product executables instead of assuming
  RivetHost.
• Make AppImage command capture check exit status and collect stderr without
  the serial-pipe deadlock risk of the previous process* implementation.
• Keep the source-package release smoke test self-contained by validating its
  JSON contract with Racket instead of an incidental Python dependency.
• Refresh the English and Chinese Linux capability summaries to reflect the
  shipped native installers and StatusNotifierItem tray implementation.