Phase 3C: Deployment pipeline & Store-and-Forward engine

Deployment Manager (WP-1–8, WP-16):
- DeploymentService: full pipeline (flatten→validate→send→track→audit)
- OperationLockManager: per-instance concurrency control
- StateTransitionValidator: Enabled/Disabled/NotDeployed transition matrix
- ArtifactDeploymentService: broadcast to all sites with per-site results
- Deployment identity (GUID + revision hash), idempotency, staleness detection
- Instance lifecycle commands (disable/enable/delete) with deduplication

Store-and-Forward (WP-9–15):
- StoreAndForwardStorage: SQLite persistence, 3 categories, no max buffer
- StoreAndForwardService: fixed-interval retry, transient-only buffering, parking
- ReplicationService: async best-effort to standby (fire-and-forget)
- Parked message management (query/retry/discard from central)
- Messages survive instance deletion, S&F drains on disable

620 tests pass (+79 new), zero warnings.

src/ScadaLink.DeploymentManager/StateTransitionValidator.cs

@@ -0,0 +1,48 @@
+using ScadaLink.Commons.Types.Enums;
+
+namespace ScadaLink.DeploymentManager;
+
+/// <summary>
+/// WP-4: State transition matrix for instance lifecycle.
+///
+/// State     | Deploy | Disable | Enable | Delete
+/// ----------|--------|---------|--------|-------
+/// NotDeploy | OK     | NO      | NO     | NO
+/// Enabled   | OK     | OK      | NO     | OK
+/// Disabled  | OK*    | NO      | OK     | OK
+///
+/// * Deploy on a Disabled instance also enables it.
+/// </summary>
+public static class StateTransitionValidator
+{
+    public static bool CanDeploy(InstanceState currentState) =>
+        currentState is InstanceState.NotDeployed or InstanceState.Enabled or InstanceState.Disabled;
+
+    public static bool CanDisable(InstanceState currentState) =>
+        currentState == InstanceState.Enabled;
+
+    public static bool CanEnable(InstanceState currentState) =>
+        currentState == InstanceState.Disabled;
+
+    public static bool CanDelete(InstanceState currentState) =>
+        currentState is InstanceState.Enabled or InstanceState.Disabled;
+
+    /// <summary>
+    /// Returns a human-readable error message if the transition is invalid, or null if valid.
+    /// </summary>
+    public static string? ValidateTransition(InstanceState currentState, string operation)
+    {
+        var allowed = operation.ToLowerInvariant() switch
+        {
+            "deploy" => CanDeploy(currentState),
+            "disable" => CanDisable(currentState),
+            "enable" => CanEnable(currentState),
+            "delete" => CanDelete(currentState),
+            _ => false
+        };
+
+        if (allowed) return null;
+
+        return $"Operation '{operation}' is not allowed when instance is in state '{currentState}'.";
+    }
+}