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/ProtocolCore.cs

@@ -79,6 +79,7 @@ internal static class JsonHelpers
         result = 0;
         if (node is not JsonValue value || value.TryGetValue<bool>(out _)) return false;
         if (value.TryGetValue<uint>(out var u) && u >= min && u <= max) { result = u; return true; }
+        if (value.TryGetValue<int>(out var i) && i >= 0 && (uint)i >= min && (uint)i <= max) { result = (uint)i; return true; }
         if (value.TryGetValue<long>(out var l) && l >= min && l <= max) { result = (uint)l; return true; }
         if (value.TryGetValue<double>(out var d) && double.IsFinite(d) && d == Math.Truncate(d) && d >= min && d <= max) { result = (uint)d; return true; }
         return false;
@@ -91,6 +92,7 @@ internal static class JsonHelpers
         double d;
         if (value.TryGetValue<double>(out var direct)) d = direct;
         else if (value.TryGetValue<long>(out var l)) d = l;
+        else if (value.TryGetValue<int>(out var iv)) d = iv;
         else if (value.TryGetValue<decimal>(out var m)) d = (double)m;
         else return false;
         if (!double.IsFinite(d) || d < min || d > max) return false;