Cross-platform packaging (cargo tauri build)

[GordonBeeming]

Description

Package the app for distribution on all three platforms.

Acceptance Criteria

• cargo tauri build produces:
  • Windows: .msi installer and .exe portable
  • macOS: .dmg with .app bundle
  • Linux: .deb, .AppImage
• C# AOT sidecar binary included in all packages
• Code signing configuration documented (certificates needed)
• Build scripts in build/ directory for each platform
• Package size target: < 30MB per platform
• CI produces release artifacts on git tag