Yavsc.Blogs.Tests will need a fixture too. Lifting the cross-cutting
plumbing (Kestrel + self-signed cert + address discovery + lazy init)
into a new Yavsc.Tests.Shared project lets the next fixture inherit
from it without copying 200+ lines of setup boilerplate, and keeps
the Org.Tests fixture focused on its IdentityServer + SMTP seed.
* New project src/Yavsc.Tests.Shared with WebHostFixture (abstract)
and TestAuthPolicyProvider (test auth bypass via X-Test-Role).
* WebServerFixture in Org.Tests now inherits from WebHostFixture;
BuildApp + ConfigurePipelineAsync hold only Org-specific work.
* Two shared package versions promoted to the root Directory.Packages.props.
* Tests still 30/30 green.
