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, 4 insertions, 3 deletions

diff --git a/src/zenutil/cloud/minioprocess.cpp b/src/zenutil/cloud/minioprocess.cpp
index 565705731..457453bd8 100644
--- a/src/zenutil/cloud/minioprocess.cpp
+++ b/src/zenutil/cloud/minioprocess.cpp
@@ -72,11 +72,12 @@ struct MinioProcess::Impl
 					ZEN_INFO("MinIO server started successfully (waited {})", NiceTimeSpanMs(Timer.GetElapsedTimeMs()));
 					return;
 				}
-			} while (Timer.GetElapsedTimeMs() < 10000);
+			} while (Timer.GetElapsedTimeMs() < 30000);
 		}
 
-		// Report failure
-		ZEN_WARN("MinIO server failed to start within timeout period");
+		// Report failure - throw so test failures show the real cause instead of a confusing
+		// assertion failure later when S3 operations fail silently.
+		throw std::runtime_error(fmt::format("MinIO server on port {} failed to start within timeout", m_Options.Port));
 	}
 
 	void StopMinioServer()