Polish release documentation and CI

[codeugar]

What changed

• turned the README into a complete project landing page with download, installation, screenshots, permissions, build, and hardware-limit guidance
• added SHA-256 checksum generation for both architecture-specific DMGs
• added macOS CI that runs the test suite, builds the universal app, verifies architectures and signing, builds both DMGs, and uploads temporary artifacts

Why

The initial public release should be understandable and independently verifiable by users and contributors. Automated checks also protect the native macOS build and release scripts from regressions.

Validation

• ./Scripts/run-tests.sh — 8/8 tests passed
• ./Scripts/build-app.sh — universal arm64 + x86_64 app built and verified
• ./Scripts/build-dmgs.sh — both DMGs built and passed hdiutil verify
• mounted both DMGs and verified their expected architecture, bundle signature, app icon, and Applications shortcut