MKV Muxing Batch GUI 2.8.0

Added

• Add automatic background and manual update checks for stable application and
  MKVToolNix releases, with actionable notifications for outdated versions.
• Add a reproducible Windows dependency fetcher that downloads MKVToolNix 101.0
  from the publisher, requires the live and repository-pinned SHA-256 values to
  agree, and extracts only the two required tools and license notices.
• Add a repository binary-policy regression test and a documented payload audit.
• Add a GitHub Actions workflow that tests and builds Windows and Linux release
  packages, validates the packaged applications, and publishes verified assets
  with a combined SHA-256 manifest from stable version tags.

Changed

• Missing MKVToolNix tools no longer prevent the application from opening. The
  startup warning can open the official download page, retry tool discovery,
  or let the user continue without muxing support.
• Windows release builds stage verified MKVToolNix tools outside the source tree;
  Linux and source builds continue to prefer a current system installation.
• Limit comtypes installation to Windows and use source-defined taskbar COM
  interfaces without a precompiled type library.

Removed

• Remove all committed MKVToolNix executables, the obsolete TaskbarLib.tlb,
  and the redundant Prepare Exe.rar release-preparation archive.