Let the fan settle downward on a re-assert

A symmetric deadband catches the fan on the way up and holds it there:
on iz-pve0 under load, 17 of 20 samples ran at 38% while the curve
wanted 34-36%, and nothing would bring it down until the request fell a
full 8 points. Damping was turning into a permanently louder machine.

A re-assert may now settle the speed downward to the current request.
Upward moves still require a full deadband crossing, so the drift this
was built to prevent cannot come back - covered by a test.

Verified under a live migration: settled 38 -> 36 on the first
re-assert, then one change in ten minutes while the request wandered
36-42, CPU stable at 66-69c and disks a degree cooler.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

monitor.sh

@@ -97,6 +97,25 @@ next_fan_speed() {
   fi
 }
 
+# settle_fan_speed <wanted> <applied> <re-assert due, 0|1>
+#
+# A symmetric deadband catches the fan on the way up and then holds it there:
+# measured on iz-pve0 under load, 17 of 20 samples ran at 38% while the curve
+# wanted 34-36%, and nothing would bring it down until the request fell a full
+# deadband. That is how damping turns into a permanently louder machine.
+#
+# So on a re-assert - and only then - let the speed settle DOWNWARD to what is
+# actually wanted. Upward moves still require a full deadband crossing, so this
+# cannot bring back the leak that let the setpoint drift 32 -> 36 -> 31.
+settle_fan_speed() {
+  local wanted=$1 applied=$2 due=$3 target
+  target=$(next_fan_speed "$wanted" "$applied")
+  if [ "$due" = 1 ] && [ -n "$applied" ] && [ "$wanted" -lt "$target" ]; then
+    target=$wanted
+  fi
+  echo "$target"
+}
+
 ALERT_EMAIL=${ALERT_EMAIL:-Servers@ntfy1.izebra.xyz}
 ALERT_COOLDOWN=${ALERT_COOLDOWN:-3600}
 STATE_DIR=${STATE_DIR:-/root/fan_speed/state}