Fix macOS DMG presentation in release builds

[BeaCox]

Summary

• replace the blank DMG background with a field-manual guided-install layout
• opt into Tauri Finder layout generation on GitHub Actions with TAURI_BUNDLER_DMG_IGNORE_CI
• mount and verify every release DMG before staging it
• add cross-platform validation tests and document the release invariant

Root cause

Tauri automatically adds --skip-jenkins when CI=true unless TAURI_BUNDLER_DMG_IGNORE_CI=true. The released DMGs therefore had no .DS_Store and opened in Finder default icon view. The tracked background was also a blank solid-color image.

Validation

• make check
• local make bundles
• new DMG mounted read-only and passed scripts/verify-macos-dmg.sh
• old Draft DMG correctly failed the new layout check
• Finder visual QA confirmed the 760 x 440 guided install layout