Unix Domain Socket auto discovery (#833)

This PR adds end-to-end Unix domain socket (UDS) support, allowing zen CLI to discover and connect to UDS-only servers automatically.

- **`unix://` URI scheme in zen CLI**: The `-u` / `--hosturl` option now accepts `unix:///path/to/socket` to connect to a zenserver via a Unix domain socket instead of TCP.
- **Per-instance shared memory for extended server info**: Each zenserver instance now publishes a small shared memory section (keyed by SessionId) containing per-instance data that doesn't fit in the fixed-size ZenServerEntry -- starting with the UDS socket path. This is a 4KB pagefile-backed section on Windows (`Global\ZenInstance_{sessionid}`) and a POSIX shared memory object on Linux/Mac (`/UnrealEngineZen_{sessionid}`).
- **Client-side auto-discovery of UDS servers**: `zen info`, `zen status`, etc. now automatically discover and prefer UDS connections when a server publishes a socket path. Servers running with `--no-network` (UDS-only) are no longer invisible to the CLI.
- **`kNoNetwork` flag in ZenServerEntry**: Servers started with `--no-network` advertise this in their shared state entry. Clients skip TCP fallback for these servers, and display commands (`ps`, `status`, `top`) show `-` instead of a port number to indicate TCP is not available.

1 files changed, 45 insertions, 1 deletions

diff --git a/src/zen/zen.cpp b/src/zen/zen.cpp
index 10e3a85fe..86154c291 100644
--- a/src/zen/zen.cpp
+++ b/src/zen/zen.cpp
@@ -369,9 +369,31 @@ GetReturnCodeFromHttpResult(const HttpClientError& Ex)
 	}
 }
 
+bool
+ZenCmdBase::IsUnixSocketSpec(std::string_view Spec)
+{
+	return Spec.starts_with("unix://");
+}
+
+HttpClient
+ZenCmdBase::CreateHttpClient(const std::string& HostSpec, HttpClientSettings Settings)
+{
+	if (IsUnixSocketSpec(HostSpec))
+	{
+		Settings.UnixSocketPath = HostSpec.substr(7);  // strip "unix://"
+		return HttpClient("http://localhost", Settings);
+	}
+	return HttpClient(HostSpec, Settings);
+}
+
 std::string
 ZenCmdBase::ResolveTargetHostSpec(const std::string& InHostSpec, uint16_t& OutEffectivePort)
 {
+	if (IsUnixSocketSpec(InHostSpec))
+	{
+		return InHostSpec;	// pass through as-is; parsed later in CreateHttpClient
+	}
+
 	if (InHostSpec.empty())
 	{
 		// If no host is specified then look to see if we have an instance
@@ -386,8 +408,30 @@ ZenCmdBase::ResolveTargetHostSpec(const std::string& InHostSpec, uint16_t& OutEf
 			Servers.Snapshot([&](const zen::ZenServerState::ZenServerEntry& Entry) {
 				if (ResolvedSpec.empty())
 				{
-					ResolvedSpec	 = fmt::format("http://localhost:{}", Entry.EffectiveListenPort.load());
 					OutEffectivePort = Entry.EffectiveListenPort;
+
+					// Check for per-instance info (e.g. UDS path)
+					if (Entry.HasInstanceInfo())
+					{
+						ZenServerInstanceInfo Info;
+						if (Info.OpenReadOnly(Entry.GetSessionId()))
+						{
+							InstanceInfoData Data = Info.Read();
+							if (!Data.UnixSocketPath.empty())
+							{
+								ResolvedSpec = "unix://" + PathToUtf8(Data.UnixSocketPath);
+								return;
+							}
+						}
+					}
+
+					// Skip servers with --no-network since TCP is not reachable
+					if (Entry.IsNoNetwork())
+					{
+						return;
+					}
+
+					ResolvedSpec = fmt::format("http://localhost:{}", Entry.EffectiveListenPort.load());
 				}
 			});