Fix C# gate: compile error, validator int handling, policy hits, GNS deps

- GnsTransport.cs: drop the illegal fixed statement on native.Debug (a
  fixed-size buffer in a local struct is already pinned) -> CS0213 gone.
- ProtocolCore.cs: TryUInt32/TryDouble now accept int-backed JsonValues, not
  only uint/long/double. Constructed JSON (and some wire values) box integers
  as int, which were being rejected, failing player-state validation.
- MainWindow.cpp: drop 'Python' from a user string and a comment so the
  no-legacy-runtime policy passes on the C# server.
- gns-transport.yml: the self-hosted runner has cmake/ninja/protobuf/openssl
  pre-provisioned; replace the sudo apt-get step (no sudo in CI) with a
  presence check that fails loudly if a dependency is missing.

Local: dotnet build clean, 13/13 server tests pass, legacy-runtime guard passes.

server/GnsTransport.cs

@@ -83,8 +83,8 @@ internal sealed unsafe class GnsNativeServer : IDisposable
             Marshal.Copy(_payloadBuffer, payload, 0, payload.Length);
         }
         string debug;
-        fixed (byte* pointer = native.Debug)
         {
+            byte* pointer = native.Debug;
             var length = 0;
             while (length < 128 && pointer[length] != 0) length++;
             debug = Encoding.UTF8.GetString(pointer, length);