Add an end-to-end TCP acceptance harness (issue #15) (#20)

Stand up the real AuthoritativeServer behind the real TCP transport on a
loopback port and drive real SyntheticProtocolClient sockets through it, so the
matrix is exercised over an actual connection rather than in-memory fakes.

Covers the baseline-protocol, TCP-compatibility, and interest-management
sections: welcome->hello->sessionReady handshake, unique/non-spoofable
server-owned ids, malformed and oversized rejection before mutation, rate-limit
tripping, transform/playerState/worldState relay, action events not
replay-cached to late joiners, disconnect/reconnect leaving no stale session,
same-cell relay vs distant interest filtering, multi-client no cross-cell spam,
and handshake-timeout reaping.

The harness runs for real wherever loopback TCP can bind (dev machines, the
self-hosted runner) and skips cleanly otherwise. Wired into a self-hosted CI
job gated by the runtime-policy guard. 60s idle-timeout, 32/64-client load /
packet-loss / reconnect-churn, and GNS sections remain follow-ups.

server/acceptance/CommonwealthOnline.Server.Acceptance.csproj

@@ -0,0 +1,14 @@
+<Project Sdk="Microsoft.NET.Sdk">
+  <PropertyGroup>
+    <OutputType>Exe</OutputType>
+    <TargetFramework>net8.0</TargetFramework>
+    <ImplicitUsings>enable</ImplicitUsings>
+    <Nullable>enable</Nullable>
+    <LangVersion>latest</LangVersion>
+    <AssemblyName>CommonwealthOnline.Server.Acceptance</AssemblyName>
+    <RootNamespace>CommonwealthOnline.Server.Acceptance</RootNamespace>
+  </PropertyGroup>
+  <ItemGroup>
+    <ProjectReference Include="../CommonwealthOnline.Server.csproj" />
+  </ItemGroup>
+</Project>