v0.7.0-alpha.3

First release published to nuget.org. No library change since
v0.7.0-alpha.1 — every diff is in the publish workflow, so this is an
iterate of the same milestone rather than a new version.

- Publishes to nuget.org via Trusted Publishing. The publish job
  exchanges its OIDC token for a short-lived key, scoped by the
  `nuget.org` environment; no publishing secret is stored.
- Stops pushing to GitHub Packages. The reseed left every FrameFlow.*
  package there owned by frame-flow-archive, and a package is scoped to
  its owning repository, so this repository's token cannot write to them.
- Asserts the package family is complete before publishing. The expected
  set is derived from the solution plus FrameFlow.Native.Runtime, so a
  partial release fails rather than ships.
- Clears packages/ before packing, so a .nupkg left by an earlier tag on
  the persistent self-hosted workspace cannot be published alongside
  this release, and requires every package to carry this tag's version.

v0.7.0-alpha.2 was tagged but never published: its run failed on the
GitHub Packages 403 described above, before reaching nuget.org.