Build a native installer upgrade and uninstall fixture matrix

[OthmaneBlial]

Context

The package matrix proves clean install, offline launch, and uninstall for six host formats. Stable releases also need evidence that an installed previous version upgrades without losing user data and that uninstall removes binaries without deleting owned data silently.

Affected area

• native package smoke workflow
• package install scripts
• release fixtures and receipts
• platform support documentation

Acceptance criteria

• Define a previous-version fixture and current-version upgrade path for app, dmg, nsis, msi, deb, and AppImage where the format supports installation.
• Verify application data survives upgrade and remains after uninstall unless the user explicitly chose deletion.
• Verify binaries, shortcuts, and registrations are removed after uninstall.
• Emit one versioned receipt per host and format with exact source versions.
• Document format-specific boundaries such as AppImage replacement rather than installer upgrade.

Test command

bash -n scripts/ci_package_install_smoke.sh
node --test scripts/assert_offline_smoke.test.mjs
gh workflow view package-verify.yml

Non-goals

• Automatic in-app updates.
• Deleting user data during uninstall.
• Claiming a format supports an installer behavior it does not provide.