feat: record RTT history per heartbeat for charting

Capture RTT on every heartbeat using the existing plugin_data history
mechanism, recording under rtt_ipv4 or rtt_ipv6 keys. Maintains 100-sample
retention per host and address family.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01DEimzMv4Q5EjFg3hoiZ69T

hbd/server/udp.py

@@ -482,6 +482,7 @@ def handle_datagram(msg: dict, addr, transport, ctx: dict):
             print("conndata failed: %s" % e)
         return
 
+
     if res:
         eventlog(uname, "WARNING", res)
         if host.watched:
@@ -562,6 +563,10 @@ def handle_datagram(msg: dict, addr, transport, ctx: dict):
     if interval > 0:
         host.interval = interval
 
+    # Record RTT history for charting
+    if rtt is not None:
+        host.add_plugin_data(f"rtt_{conn.afam.lower()}", {"rtt": rtt}, timestamp=now)
+
     # Timer-based reachability monitoring
     # Reset overdue timer on every heartbeat
     if interval > 0 and conn.getstate() != hbdcls.Connection.DOWN: