HackDigest v0.2.0

Linux packaging parity: the Linux line gets first-class installer assets,
matching the family bar on macOS and Windows. Nothing about the update feed
or the in-app updater changes — these are installer assets published beside
the portable tar.gz.

Added

• Linux .deb packages for x64 and arm64, hand-rolled with dpkg-deb:
  the app installs to /opt/hackdigest with a hackdigest launcher on
  PATH, a desktop menu entry, and hicolor icons. Depends names what
  the runtime actually FFI-loads (WebKitGTK, GTK 3, OpenSSL 3 for the
  updater's signature checks, xdg-utils) instead of a laundry list, and
  release CI installs the package into a clean ubuntu:22.04 container
  to prove the dependency set is sufficient.
• Linux AppImage (x64), assembled with appimagetool from the same
  distribution directory — same system-WebKitGTK requirement as the
  tar.gz and deb, so it stays honest about what it bundles.
• Linux arm64 builds: the release pipeline now runs its Linux leg on a
  native arm64 runner, shipping hackdigest-<version>-linux-arm64.tar.gz
  and .deb beside the x64 ones. Arm64 is an installer asset only for
  now — the signed update feed still carries the x64 tar.gz.

Changed

• Release assets follow the family naming on Linux too:
  hackdigest-<version>-linux-<arch>.<ext> now covers .tar.gz, .deb
  and .AppImage, all covered by SHA256SUMS and build-provenance
  attestations.