fix: preserve chart history across reconnects, show gaps for missing data

Only wipe real (non-RTT) plugin data on an actual client reboot (boot
flag), not on every ordinary OVERDUE/DOWN -> UP recovery. A transient
network blip no longer erases CPU/memory/etc. history.

Also split the shared time-series chart into separate line/area segments
wherever the gap between samples is much larger than the typical spacing,
so missing data (host overdue, or history that simply hasn't accumulated
across a drop) renders as a visual gap instead of an interpolated line.

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

hbd/server/udp.py

@@ -521,13 +521,17 @@ def handle_datagram(msg: dict, addr, transport, ctx: dict):
         # Transition to UP and log/notify if appropriate
         lasts = conn.state
         d = conn.newstate(hbdcls.Connection.UP, now)
-        # On reboot, pre-boot plugin data and derived alerts are stale.
-        # Cancel all plugin timers and wipe plugin state so timers restart
-        # cleanly from the first two post-boot samples.
-        for pname in list(host.plugin_timers):
-            host.cancel_plugin_timer(pname)
-        for pname in [k for k in host.plugin_data if not _is_rtt_key(k)]:
-            del host.plugin_data[pname]
+        if boot:
+            # On reboot, pre-boot plugin data and derived alerts are stale.
+            # Cancel all plugin timers and wipe plugin state so timers restart
+            # cleanly from the first two post-boot samples. An ordinary
+            # reconnect (no boot flag) doesn't invalidate the client's
+            # already-collected data, so it's left alone — this keeps chart
+            # history intact across a transient network blip.
+            for pname in list(host.plugin_timers):
+                host.cancel_plugin_timer(pname)
+            for pname in [k for k in host.plugin_data if not _is_rtt_key(k)]:
+                del host.plugin_data[pname]
         stale_plugin_keys = [
             k for k in host.alert_states
             if k not in ("rtt",) and not k.startswith("connectivity.")