fix(health): route site heartbeats into the aggregator

CentralCommunicationActor.HandleHeartbeat was forwarding each incoming
HeartbeatMessage to Context.Parent, which resolves to the /user
guardian — a non-actor. Every site heartbeat went straight to dead
letters (~1026 per central node per 30 minutes at the default ~2s
interval across three sites).

The aggregator now exposes MarkHeartbeat(siteId, receivedAt) which
bumps LastReportReceivedAt on already-known sites (and clears IsOnline
if it had flipped) without touching LatestReport. Heartbeats from
unregistered sites are dropped — first registration still happens on
the first full report. CentralCommunicationActor calls this in place
of the no-op Tell.

The result: heartbeats now serve their stated health-monitoring
purpose (per CLAUDE.md) by keeping a site marked online between the
30s full reports if a single report is briefly delayed, and the dead
letter noise disappears entirely.

tests/ScadaLink.Communication.Tests/ScadaLink.Communication.Tests.csproj

@@ -25,6 +25,7 @@
   <ItemGroup>
     <ProjectReference Include="../../src/ScadaLink.Communication/ScadaLink.Communication.csproj" />
     <ProjectReference Include="../../src/ScadaLink.Commons/ScadaLink.Commons.csproj" />
+    <ProjectReference Include="../../src/ScadaLink.HealthMonitoring/ScadaLink.HealthMonitoring.csproj" />
   </ItemGroup>
 
 </Project>