ZFS tab: Writes — the RAM write buffer, each pool's write rate, and a landing tier with its mover

Live from arcstats (dirty), dmu_tx (throttled writes), each pool's txgs (bytes synced to disk)
and objset kstats (bytes from programs). A landing tier — a mergerfs pool whose first branch is
fast storage and whose last is a ZFS dataset — is detected from mergerfs's branch xattr; a mover
status file in /run/landing-mover/ adds its queue, the file in flight and totals. Sensors
zfs.write.*, zfs.<pool>.write_bps|logical_bps and zfs.<pool>.landing.* carry the history.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

internal/metrics/metrics.go

@@ -150,7 +150,8 @@ type Sample struct {
 	FS       []FS       `json:"fs"`
 	UPS      *UPS       `json:"ups"`
 	GPUs     []GPU      `json:"gpus"`
-	ARC      *ARC       `json:"arc"` // the ZFS read cache; nil without the module
+	ARC      *ARC       `json:"arc"`        // the ZFS read cache; nil without the module
+	ZFSWrite *ZFSWrites `json:"zfs_writes"` // the ZFS write path; nil without the module
 	Uptime   float64    `json:"uptime"`
 
 	fsT, upsT int64 // when the slow sections were really read; sensors report these, not T
@@ -182,6 +183,7 @@ type Collector struct {
 	gpu       gpuState
 	power     cpuPowerState
 	prevARC   arcPrev
+	prevZW    zfsWritePrev
 
 	subMu sync.Mutex
 	subs  map[chan struct{}]struct{} // told after every sample; see Subscribe
@@ -368,6 +370,7 @@ func (c *Collector) sample() {
 	s.UPS, s.upsT = c.lastUPS, c.upsAt.UnixMilli()
 	s.GPUs = c.gpus(now, cpuTemp(s.Temps))
 	s.ARC = c.arc(now)
+	s.ZFSWrite = c.zfsWrites(now, s.ARC, s.Disks)
 
 	if first {
 		return // deltas are meaningless on the priming pass