[HexCodeYT]

Summary

• Add a shared scripts/release-version.sh helper so packaging has one version source: PLAINLINK_RELEASE_VERSION, exact Git tag, then Cargo version fallback.
• Update macOS package/release scripts and GitHub release publishing so preview tags produce preview-named artifacts.
• Let preview releases fall back from tag-specific notes to the base release notes.
• Bump GitHub Actions maintenance versions to actions/checkout@v7 and actions/upload-artifact@v7.
• Add release-script tests and refresh release docs/README/roadmap wording.

Root Cause

Preview builds and publishing used different version sources. Packaging used the Cargo version, while publishing derived artifact names from the Git tag. That made v0.1.0-preview.1 expect files that the package script never produced.

Validation

• cargo fmt --check
• cargo test
• cargo clippy --all-targets -- -D warnings
• cargo run --bin plainlink-rules -- verify-fixtures
• git diff --check
• PLAINLINK_RELEASE_VERSION=v0.1.0-preview.2 scripts/package-macos-app.sh