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

PrivaPub.Tests/PrivaPub.Tests.csproj

@@ -0,0 +1,29 @@
+<Project Sdk="Microsoft.NET.Sdk">
+
+  <PropertyGroup>
+    <TargetFramework>net10.0</TargetFramework>
+    <Nullable>disable</Nullable>
+    <ImplicitUsings>enable</ImplicitUsings>
+    <OutputType>Exe</OutputType>
+    <IsPackable>false</IsPackable>
+  </PropertyGroup>
+
+  <ItemGroup>
+    <PackageReference Include="Microsoft.NET.Test.Sdk" Version="18.10.1" />
+    <PackageReference Include="xunit.v3" Version="3.2.2" />
+    <PackageReference Include="xunit.runner.visualstudio" Version="3.1.5" />
+  </ItemGroup>
+
+  <ItemGroup>
+    <Using Include="Xunit" />
+  </ItemGroup>
+
+  <ItemGroup>
+    <ProjectReference Include="..\PrivaPub\PrivaPub.csproj" />
+  </ItemGroup>
+
+  <ItemGroup>
+    <None Update="Fixtures\**\*" CopyToOutputDirectory="PreserveNewest" />
+  </ItemGroup>
+
+</Project>