nfs-mount-monitor: add write-probe to detect 'reads OK, writes hang' state

Stunnel-wrapped NFSv4 can enter a half-broken state where mountpoint(1)
returns true and stat(1) completes from cache, but ALL writes hang
indefinitely. This was observed on r2 on 2026-05-10 causing navidrome
to be unschedulable. The existing two probes passed while writes were
dead.

Add a third probe (write-probe) after the stat probe: write the shell's
PID to a per-host .healthcheck.<hostname> file and immediately remove it,
wrapped in a 5-second timeout. The per-host filename prevents r0/r1/r2
from racing on the same file. 5s gives one full NFS retransmit window
(timeo=10 deciseconds = 1s, retrans=2) plus margin without making the
10-second timer run too long.

Deployed to r0/r1/r2 via rex nfs_mount_monitor; all three nodes
confirmed running the new script (journalctl shows clean exits).

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

0 files changed, 0 insertions, 0 deletions