ci: bring back stable releases on v* tag push

Local stable builds picked up host CPU flags (AVX-512) and shipped
binaries that crashed on user CPUs without those instructions. CI runs
on generic GitHub-hosted x86-64 runners, so all release builds need to
go through it now.

- Trigger workflow on push of any v* tag in addition to main.
- Resolve channel from the ref: tags/v* -> stable, otherwise beta.
- Tarball name follows channel: fluorine-manager-<version>.tar.gz on
  stable, fluorine-manager-beta.tar.gz on beta.
- Beta publish step gated on channel==beta to avoid double-publishing
  when a tag also lives on main.
- New "Publish stable release" step creates/updates a release at the
  tag, marks it --latest, and attaches the versioned tarball.

To cut v0.2.0: `git tag v0.2.0 && git push origin v0.2.0`.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

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