Fix false presence: only trigger arrival on NUD_REACHABLE, not STALE/DELAY/PROBE

Kernel fires RTM_NEWNEIGH for NUD_STALE, NUD_DELAY, and NUD_PROBE states
as it probes a potentially-departed device. Each probe was calling on_arrival()
which refreshed last_seen, preventing the watchdog from ever detecting departure.

Only NUD_REACHABLE confirms a device is actually on the network.

net_alerter/net_alerter.py

@@ -1738,7 +1738,7 @@ def parse_ndmsg(data: bytes, msg_type: int) -> None:
         if not mac or mac in ('00:00:00:00:00:00', 'ff:ff:ff:ff:ff:ff'):
             return
 
-        if msg_type == RTM_NEWNEIGH and (state & (NUD_REACHABLE | NUD_STALE | NUD_DELAY | NUD_PROBE)):
+        if msg_type == RTM_NEWNEIGH and (state & NUD_REACHABLE):
             on_arrival(mac, ip or 'unknown')
         elif msg_type == RTM_DELNEIGH:
             on_departure(mac)