Subprocess Manager (#889)

Adds a `SubprocessManager` for managing child processes with ASIO-integrated async exit detection, stdout/stderr pipe capture, and periodic metrics sampling. Also introduces `ProcessGroup` for OS-backed process grouping (Windows JobObjects / POSIX process groups).

### SubprocessManager
- Async process exit detection using platform-native mechanisms (Windows `object_handle`, Linux `pidfd_open`, macOS `kqueue EVFILT_PROC`) — no polling
- Stdout/stderr capture via async pipe readers with per-process or default callbacks
- Periodic round-robin metrics sampling (CPU, memory) across managed processes
- Spawn, adopt, remove, kill, and enumerate managed processes

### ProcessGroup
- OS-level process grouping: Windows JobObject (kill-on-close guarantee), POSIX `setpgid` (bulk signal delivery)
- Atomic group kill via `TerminateJobObject` (Windows) or `kill(-pgid, sig)` (POSIX)
- Per-group aggregate metrics and enumeration

### ProcessHandle improvements
- Added explicit constructors from `int` (pid) and `void*` (native handle)
- Added move constructor and move assignment operator

### ProcessMetricsTracker
- Cross-platform process metrics (CPU time, working set, page faults) via `QueryProcessMetrics()`
- ASIO timer-driven periodic sampling with configurable interval and batch size
- Aggregate metrics across tracked processes

### Other changes
- Fixed `zentest-appstub` writing a spurious `Versions` file to cwd on every invocation

1 files changed, 53 insertions, 0 deletions

diff --git a/src/zencore/process.cpp b/src/zencore/process.cpp
index dcb8b2422..e7baa3f8e 100644
--- a/src/zencore/process.cpp
+++ b/src/zencore/process.cpp
@@ -277,6 +277,46 @@ CreateStdoutPipe(StdoutPipeHandles& OutPipe)
 
 ProcessHandle::ProcessHandle() = default;
 
+ProcessHandle::ProcessHandle(int Pid)
+{
+	Initialize(Pid);
+}
+
+#if ZEN_PLATFORM_WINDOWS
+ProcessHandle::ProcessHandle(void* NativeHandle)
+{
+	Initialize(NativeHandle);
+}
+#endif
+
+ProcessHandle::ProcessHandle(ProcessHandle&& Other) noexcept
+: m_ProcessHandle(Other.m_ProcessHandle)
+, m_Pid(Other.m_Pid)
+#if ZEN_PLATFORM_LINUX || ZEN_PLATFORM_MAC
+, m_ExitCode(Other.m_ExitCode)
+#endif
+{
+	Other.m_ProcessHandle = nullptr;
+	Other.m_Pid			  = 0;
+}
+
+ProcessHandle&
+ProcessHandle::operator=(ProcessHandle&& Other) noexcept
+{
+	if (this != &Other)
+	{
+		Reset();
+		m_ProcessHandle = Other.m_ProcessHandle;
+		m_Pid			= Other.m_Pid;
+#if ZEN_PLATFORM_LINUX || ZEN_PLATFORM_MAC
+		m_ExitCode = Other.m_ExitCode;
+#endif
+		Other.m_ProcessHandle = nullptr;
+		Other.m_Pid			  = 0;
+	}
+	return *this;
+}
+
 #if ZEN_PLATFORM_WINDOWS
 void
 ProcessHandle::Initialize(void* ProcessHandle)
@@ -720,6 +760,10 @@ CreateProcNormal(const std::filesystem::path& Executable, std::string_view Comma
 	}
 	if (Options.Flags & CreateProcOptions::Flag_NoConsole)
 	{
+		CreationFlags |= DETACHED_PROCESS;
+	}
+	if (Options.Flags & CreateProcOptions::Flag_NoWindow)
+	{
 		CreationFlags |= CREATE_NO_WINDOW;
 	}
 	if (Options.Flags & CreateProcOptions::Flag_Windows_NewProcessGroup)
@@ -930,6 +974,10 @@ CreateProcUnelevated(const std::filesystem::path& Executable, std::string_view C
 	}
 	if (Options.Flags & CreateProcOptions::Flag_NoConsole)
 	{
+		CreateProcFlags |= DETACHED_PROCESS;
+	}
+	if (Options.Flags & CreateProcOptions::Flag_NoWindow)
+	{
 		CreateProcFlags |= CREATE_NO_WINDOW;
 	}
 	if (AssignToJob)
@@ -1070,6 +1118,11 @@ CreateProc(const std::filesystem::path& Executable, std::string_view CommandLine
 			}
 		}
 
+		if (Options.ProcessGroupId > 0)
+		{
+			setpgid(0, Options.ProcessGroupId);
+		}
+
 		for (const auto& [Key, Value] : Options.Environment)
 		{
 			setenv(Key.c_str(), Value.c_str(), 1);