v0.7.0-alpha.2

First release published to nuget.org. No library change from
v0.7.0-alpha.1 — the only diff is the workflow that publishes it, so
this is an iterate of the same milestone rather than a new version.

- Adds the nuget.org publish path 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.
- Adds a fail-loud completeness gate. The expected package set is
  derived from FrameFlow.slnx plus FrameFlow.Native.Runtime, so a
  partial release fails rather than ships.
- Clears packages/ before packing. The self-hosted workspace outlives
  the job, so a .nupkg from an earlier tag could otherwise be published
  alongside this release.
- Asserts every package carries this tag's version.

FrameFlow.Native.Runtime reaching nuget.org is the point: six projects
take a PackageReference on it, and until now it existed only in a
private feed, so a public clone could not restore them.