Advisory, single-writer lock on a database directory (ADR-hjxp/0006), claimed with
O_CREAT|O_EXCL and kept alive by a heartbeat that is written to a fresh temporary and renamed
over the lock file (ADR-6wjp/0016), so a stale timestamp is never mistaken for proof that the
holder is gone and no write ever follows a link somebody planted in the way.
A lease a caller has already challenged and waited the hand-over grace out on, in the shape
that survives a trip out to a host and back: the owner token as the number its hex spells, and
the heartbeat that token had not moved past.
The contents of .hayson.lock. owner is the only field ownership is decided on; pid and
host are diagnostics, present on unix and absent on platforms without processes.