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, 48 insertions, 0 deletions

diff --git a/src/zenutil/process/exitwatcher.h b/src/zenutil/process/exitwatcher.h
new file mode 100644
index 000000000..24906d7d0
--- /dev/null
+++ b/src/zenutil/process/exitwatcher.h
@@ -0,0 +1,48 @@
+// Copyright Epic Games, Inc. All Rights Reserved.
+
+#pragma once
+
+#include <zencore/process.h>
+#include <zencore/zencore.h>
+
+#include <functional>
+#include <memory>
+
+namespace asio {
+class io_context;
+}
+
+namespace zen {
+
+/// Async process exit watcher.
+///
+/// Uses platform-specific mechanisms for scalable, non-polling exit detection:
+///   Linux:   pidfd_open() + asio::posix::stream_descriptor
+///   Windows: asio::windows::object_handle
+///   macOS:   kqueue EVFILT_PROC/NOTE_EXIT + asio::posix::stream_descriptor
+///
+/// The callback is invoked exactly once when the process exits, posted to the
+/// io_context. Call Cancel() to suppress the callback.
+class ProcessExitWatcher
+{
+public:
+	explicit ProcessExitWatcher(asio::io_context& IoContext);
+	~ProcessExitWatcher();
+
+	ProcessExitWatcher(const ProcessExitWatcher&) = delete;
+	ProcessExitWatcher& operator=(const ProcessExitWatcher&) = delete;
+
+	/// Begin watching the given process. The callback is posted to the io_context
+	/// when the process exits. Only one Watch() may be active at a time.
+	void Watch(const ProcessHandle& Handle, std::function<void(int ExitCode)> OnExit);
+
+	/// Cancel any outstanding watch. The callback will not be invoked after this
+	/// returns. Safe to call if no watch is active.
+	void Cancel();
+
+private:
+	struct Impl;
+	std::unique_ptr<Impl> m_Impl;
+};
+
+}  // namespace zen