Subprocess Manager (#889)HEADmain

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

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