Export system and node editor UI are complete; piano roll editor is in progress.
Add a "Download a prebuilt release" section (GitHub releases) for users who don't
want to build from source, and nest the build-from-source steps under their own heading.
rust-ui is being merged into master and retired as the primary branch.
Update README, ARCHITECTURE, and CONTRIBUTING to reference master.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Skyler Lehmkuhl