Automate versioned releases

[EricSun0218]

What changed

• add a staged GitHub Release and NuGet OIDC publication pipeline
• build versioned Godot, Unity, portable server, and NuGet assets with SHA256 checksums
• include the complete Apache-2.0 license in standalone engine packages
• document NuGet and engine archive installation in both READMEs

Why

The public source repository did not yet provide permanent versioned downloads or package-manager distribution. Standalone engine archives also needed to carry the complete license outside the repository checkout.

Validation

• 219 tests passed in the release build
• real engine package gates passed for Godot and Unity
• six NuGet packages restored and built in a fresh consumer
• release archives and checksums were regenerated and independently verified
• all workflows passed actionlint 1.7.12
• public-tree and staged-diff checks passed