ci(desktop): guard Windows installer artifacts

[avs-io]

Problem

The desktop release pipeline did not enforce that Windows NSIS installer artifacts (.exe) were produced and verified alongside macOS and Linux builds, leaving distribution asset availability open to platform build drops.

Root cause

The GitHub Actions workflow lacked a strict verification step ensuring .exe artifacts were built and signed before publishing release assets, and release documentation did not state the mandatory asset set.

Change

• Add CI guard .github/workflows/build-windows-installer.yml to build and verify Windows NSIS installer artifacts.
• Implement strict asset verifier app/scripts/verify-windows-installer.mjs and comprehensive unit tests app/scripts/verify-windows-installer.test.ts.
• Update RELEASING.md and app/DISTRIBUTION.md to document mandatory Windows installer verification.

User impact

Release assets for desktop releases are guaranteed to include verified .exe installers for Windows users.

Preservation and out of scope

• macOS and Linux distribution workflows remain unchanged.

Testing

• Unit tests in app/scripts/verify-windows-installer.test.ts pass.
• npx tsc --noEmit clean.
• All 2600+ root tests pass.