Process management improvements (#881)

This PR improves process lifecycle handling and resilience across several areas:

- **Reclaim stale shared-memory entries instead of exiting** (`zenserver.cpp`): When a zenserver instance fails to attach as a sponsor to an existing process (e.g. because the PID was reused by an unrelated process), the server now clears the stale shared-memory entry and proceeds with normal startup instead of calling `std::exit(1)`.

- **Wait for child process exit in `Kill()` and `Terminate()` on Unix** (`process.cpp`): After sending `SIGTERM` in `Kill()`, the code now waits up to 5s for graceful shutdown (escalating to `SIGKILL` on timeout), matching the Windows behavior. `Terminate()` also waits after `SIGKILL` so the child is properly reaped and doesn't linger as a zombie clogging up the process table.

- **Fix sysctl buffer race in macOS `FindProcess`** (`process.cpp`): The macOS process enumeration now retries the `sysctl` call (up to 3 attempts with 25% buffer padding) to handle the race where the process list changes between the sizing call and the data-fetching call. Also flattens the nesting and fixes the guard/free scoping.

- **Terminate stale processes before integration tests** (`zenserver-test.cpp`, `test.lua`): The integration test runner now accepts a `--kill-stale-processes` flag (passed automatically by `test.lua`) that scans for and terminates any leftover `zenserver`, `zenserver-test`, and `zentest-appstub` processes from previous test runs, logging the executable name and PID of each. This addresses flaky test failures caused by stale processes from prior runs holding ports or other resources.

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