Replace three silently-dropped Result cases in heartbeat::write_atomic
(create_dir_all, fs::write, fs::rename) with log::error! diagnostics.
Brings heartbeat.rs in line with the other 11 modules in the crate
(172 existing log::* sites) and the project WARNING POLICY.
Also removes the orphaned temp file when rename fails (e.g. cross-fs
EXDEV) so leftover .tmp files do not accumulate across restarts.
Success-path behavior is unchanged; the 3 existing heartbeat unit
tests still pass.
vasilito
ad2a8ee15d