Every outbound federation request goes through one guarded client

Infrastructure/Http adds the client the roadmap's S3 and S4 ask for:
- the connect callback resolves the name itself and refuses loopback,
  private, link-local, CGNAT, documentation, multicast, ULA, NAT64, 6to4,
  Teredo and IPv4-mapped/compatible forms, then connects to the vetted
  address, so DNS rebinding cannot swap it afterwards;
- redirects are followed by hand, at most three, each one re-checked;
- bodies are capped at 1 MB after decompression, only JSON media types are
  read, every request has a 15 s budget, and a refused URL is not asked
  again for five minutes.

Actor and WebFinger fetches and inbox deliveries all use it. Test networks
can switch on Federation:AllowPrivateNetworks/AllowPlainHttp; startup
refuses both in Production.

PrivaPub.Tests (xUnit v3) starts with the address table and the fetcher's
limits against an in-process peer; build.yml and deploy.yml run it.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_012CzABvBkbcFqoHdmi8b9WB

.gitea/workflows/build.yml

@@ -14,3 +14,6 @@ jobs:
 
       - name: Build
         run: dotnet build PrivaPub.sln -c Release
+
+      - name: Test
+        run: dotnet test PrivaPub.sln -c Release --no-build