fix: detect recycled macOS lock PIDs

[AaronZ345]

Summary

• Add a Darwin process-identity check before treating a live PID as a valid OpenViking data-directory lock holder.
• Treat macOS stale locks as recoverable when the recorded PID has been recycled by an unrelated process, matching the existing Linux intent more closely.
• Add a regression test for a reused macOS PID held by mdwrite and adjust lock-error tests to avoid relying on real PID 1 semantics.

Root cause

_is_pid_alive() only validates process identity through /proc/<pid>/cmdline on Linux. macOS has no /proc, so a stale .openviking.pid left after reboot can be mistaken for an active OpenViking process when Darwin reuses that PID for a system daemon.

Fixes #4210.

Test plan

• OV_PREBUILT_BIN_DIR=/tmp/openviking-ov-prebuilt OV_SKIP_RAGFS_BUILD=1 uv run --extra test pytest --no-cov tests/unit/test_process_lock.py → 38 passed, 4 warnings

Note: plain uv run --extra test pytest tests/unit/test_process_lock.py could not build locally because this machine does not have Cargo available to build openviking/bin/ov. I used a temporary prebuilt ov placeholder only for local Python test execution.

[AaronZ345]

Closing this draft because #4229 already covers the same issue with a broader, earlier implementation. Avoiding duplicate reviewer load.