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, 5 insertions, 20 deletions

diff --git a/src/zenserver-test/process-tests.cpp b/src/zenserver-test/process-tests.cpp
index 649f24f54..ae11bb294 100644
--- a/src/zenserver-test/process-tests.cpp
+++ b/src/zenserver-test/process-tests.cpp
@@ -67,10 +67,7 @@ TEST_CASE("pipe.capture_stdout")
 	CreateProcOptions Options;
 	Options.StdoutPipe = &Pipe;
 
-	CreateProcResult ProcResult = CreateProc(AppStub, CommandLine, Options);
-
-	ProcessHandle Process;
-	Process.Initialize(ProcResult);
+	ProcessHandle Process(CreateProc(AppStub, CommandLine, Options));
 
 	// Close the write end, then drain before Wait() to avoid deadlock if output fills the pipe buffer.
 	Pipe.CloseWriteEnd();
@@ -95,10 +92,7 @@ TEST_CASE("pipe.capture_multiline")
 	CreateProcOptions Options;
 	Options.StdoutPipe = &Pipe;
 
-	CreateProcResult ProcResult = CreateProc(AppStub, CommandLine, Options);
-
-	ProcessHandle Process;
-	Process.Initialize(ProcResult);
+	ProcessHandle Process(CreateProc(AppStub, CommandLine, Options));
 
 	Pipe.CloseWriteEnd();
 
@@ -211,10 +205,7 @@ TEST_CASE("pipe.capture_with_nonzero_exit")
 	CreateProcOptions Options;
 	Options.StdoutPipe = &Pipe;
 
-	CreateProcResult ProcResult = CreateProc(AppStub, CommandLine, Options);
-
-	ProcessHandle Process;
-	Process.Initialize(ProcResult);
+	ProcessHandle Process(CreateProc(AppStub, CommandLine, Options));
 
 	Pipe.CloseWriteEnd();
 
@@ -237,10 +228,7 @@ TEST_CASE("pipe.stderr_on_shared_pipe")
 	CreateProcOptions Options;
 	Options.StdoutPipe = &Pipe;
 
-	CreateProcResult ProcResult = CreateProc(AppStub, CommandLine, Options);
-
-	ProcessHandle Process;
-	Process.Initialize(ProcResult);
+	ProcessHandle Process(CreateProc(AppStub, CommandLine, Options));
 
 	Pipe.CloseWriteEnd();
 
@@ -268,10 +256,7 @@ TEST_CASE("pipe.separate_stderr")
 	Options.StdoutPipe = &StdoutPipe;
 	Options.StderrPipe = &StderrPipe;
 
-	CreateProcResult ProcResult = CreateProc(AppStub, CommandLine, Options);
-
-	ProcessHandle Process;
-	Process.Initialize(ProcResult);
+	ProcessHandle Process(CreateProc(AppStub, CommandLine, Options));
 
 	StdoutPipe.CloseWriteEnd();
 	StderrPipe.CloseWriteEnd();