Verify component designs against Akka.NET best practices documentation

Cluster Infrastructure: add min-nr-of-members=1 requirement for single-node
operation after failover. Add graceful shutdown / CoordinatedShutdown section
for fast singleton handover during planned maintenance.

Site Runtime: add explicit supervision strategies per actor type (Resume for
coordinators, Stop for short-lived execution actors). Stagger Instance Actor
startup to prevent reconnection storms. Add Tell-vs-Ask usage guidance per
Akka.NET best practices (Tell for hot path, Ask for system boundaries only).

Data Connection Layer: add Connection Actor Model section documenting the
Become/Stash pattern for connection lifecycle state machine.

Health Monitoring: add dead letter count as a monitored metric.

Host: add REQ-HOST-8a for dead letter monitoring (subscribe to EventStream,
log at Warning level, report as health metric).

Component-HealthMonitoring.md

@@ -30,6 +30,7 @@ Site clusters (metric collection and reporting). Central cluster (aggregation an
 | Script error rates | Site Runtime (Script Actors) | Frequency of script failures |
 | Alarm evaluation error rates | Site Runtime (Alarm Actors) | Frequency of alarm evaluation failures |
 | Store-and-forward buffer depth | Store-and-Forward Engine | Pending messages by category (external, notification, DB write) |
+| Dead letter count | Akka.NET EventStream | Messages sent to actors that no longer exist — indicates stale references or timing issues |
 
 ## Reporting Protocol