Add automated release workflow for GitHub Actions

[VPDPersonal]

Summary

• Added .github/workflows/release.yml to automate the release process for tagged versions and manual dispatch
• Workflow validates version format (SemVer), checks package.json alignment, extracts CHANGELOG entries, builds generators, and publishes UPM subtree
• Supports both tag-triggered releases and manual workflow dispatch with custom version input
• Creates immutable per-version UPM tags and updates the upm branch for package manager consumption

Notes for review

The workflow enforces several pre-release checks:

• Version validation: Ensures SemVer format and detects pre-releases (e.g., 1.0.0-rc.2)
• package.json sync: Verifies the version in package.json matches the release tag
• CHANGELOG requirement: Extracts the release notes section; fails if missing
• Generator DLL verification: Confirms the compiled generator is present in the package before release
• UPM publishing: Uses git subtree split to publish the package contents to immutable upm/VERSION tags and the upm branch for Package Manager consumption

The workflow can be triggered by:

1. Pushing a tag matching v* (e.g., v1.0.0)
2. Manual dispatch via GitHub Actions UI with a custom version string

Pre-release versions (containing -) are automatically marked as GitHub pre-releases.