Phase 3A: Site runtime foundation — Akka cluster, SQLite persistence, Deployment Manager singleton, Instance Actor

- WP-1: Site cluster config (keep-oldest SBR, down-if-alone, 2s/10s failure detection)
- WP-2: Site-role host bootstrap (no Kestrel, SQLite paths)
- WP-3: SiteStorageService with deployed_configurations + static_attribute_overrides tables
- WP-4: DeploymentManagerActor as cluster singleton with staggered Instance Actor creation,
  OneForOneStrategy/Resume supervision, deploy/disable/enable/delete lifecycle
- WP-5: InstanceActor with attribute state, GetAttribute/SetAttribute, SQLite override persistence
- WP-6: CoordinatedShutdown verified for graceful singleton handover
- WP-7: Dual-node recovery (both seed nodes, min-nr-of-members=1)
- WP-8: 31 tests (storage CRUD, actor lifecycle, supervision, negative checks)
389 total tests pass, zero warnings.

src/ScadaLink.Commons/Messages/Instance/GetAttributeRequest.cs

@@ -0,0 +1,22 @@
+namespace ScadaLink.Commons.Messages.Instance;
+
+/// <summary>
+/// Request to get the current value of an attribute from an Instance Actor.
+/// Uses the Ask pattern for system boundaries; Tell pattern is preferred for hot-path.
+/// </summary>
+public record GetAttributeRequest(
+    string CorrelationId,
+    string InstanceUniqueName,
+    string AttributeName,
+    DateTimeOffset Timestamp);
+
+/// <summary>
+/// Response containing the current value of an attribute.
+/// </summary>
+public record GetAttributeResponse(
+    string CorrelationId,
+    string InstanceUniqueName,
+    string AttributeName,
+    object? Value,
+    bool Found,
+    DateTimeOffset Timestamp);

src/ScadaLink.Commons/Messages/Instance/SetStaticAttributeCommand.cs

@@ -0,0 +1,23 @@
+namespace ScadaLink.Commons.Messages.Instance;
+
+/// <summary>
+/// Command to set a static attribute value on an Instance Actor.
+/// Updates in-memory state and persists the override to SQLite.
+/// </summary>
+public record SetStaticAttributeCommand(
+    string CorrelationId,
+    string InstanceUniqueName,
+    string AttributeName,
+    string Value,
+    DateTimeOffset Timestamp);
+
+/// <summary>
+/// Response confirming that a static attribute was set.
+/// </summary>
+public record SetStaticAttributeResponse(
+    string CorrelationId,
+    string InstanceUniqueName,
+    string AttributeName,
+    bool Success,
+    string? ErrorMessage,
+    DateTimeOffset Timestamp);