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-DataConnectionLayer.md

@@ -67,6 +67,16 @@ Each value update delivered to an Instance Actor includes:
 - **Quality**: Data quality indicator (good, bad, uncertain).
 - **Timestamp**: When the value was read from the device.
 
+## Connection Actor Model
+
+Each data connection is managed by a dedicated connection actor that uses the Akka.NET **Become/Stash** pattern to model its lifecycle as a state machine:
+
+- **Connecting**: The actor attempts to establish the connection. Subscription requests and write commands received during this phase are **stashed** (buffered in the actor's stash).
+- **Connected**: The actor is actively servicing subscriptions. On entering this state, all stashed messages are unstashed and processed.
+- **Reconnecting**: The connection was lost. The actor transitions back to a connecting-like state, stashing new requests while it retries.
+
+This pattern ensures no messages are lost during connection transitions and is the standard Akka.NET approach for actors with I/O lifecycle dependencies.
+
 ## Connection Lifecycle & Reconnection
 
 The DCL manages connection lifecycle automatically: