Add StellaOps.Workflow engine: 14 libraries, WebService, 8 test projects

Extract product-agnostic workflow engine from Ablera.Serdica.Workflow into
standalone StellaOps.Workflow.* libraries targeting net10.0.

Libraries (14):
- Contracts, Abstractions (compiler, decompiler, expression runtime)
- Engine (execution, signaling, scheduling, projections, hosted services)
- ElkSharp (generic graph layout algorithm)
- Renderer.ElkSharp, Renderer.ElkJs, Renderer.Msagl, Renderer.Svg
- Signaling.Redis, Signaling.OracleAq
- DataStore.MongoDB, DataStore.PostgreSQL, DataStore.Oracle

WebService: ASP.NET Core Minimal API with 22 endpoints

Tests (8 projects, 109 tests pass):
- Engine.Tests (105 pass), WebService.Tests (4 E2E pass)
- Renderer.Tests, DataStore.MongoDB/Oracle/PostgreSQL.Tests
- Signaling.Redis.Tests, IntegrationTests.Shared

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

docs/workflow/tutorials/02-service-tasks/csharp/ServiceTaskWorkflow.cs

@@ -0,0 +1,83 @@
+using System.Collections.Generic;
+using System.Text.Json;
+
+using WorkflowEngine.Abstractions;
+using WorkflowEngine.Contracts;
+
+namespace WorkflowEngine.Tutorials;
+
+public sealed class PolicyValidationRequest
+{
+    public long PolicyId { get; set; }
+}
+
+public sealed class PolicyValidationWorkflow : IDeclarativeWorkflow<PolicyValidationRequest>
+{
+    public string WorkflowName => "PolicyValidation";
+    public string WorkflowVersion => "1.0.0";
+    public string DisplayName => "Policy Validation";
+    public IReadOnlyCollection<string> WorkflowRoles => ["DBA"];
+
+    public WorkflowSpec<PolicyValidationRequest> Spec { get; } = WorkflowSpec
+        .For<PolicyValidationRequest>()
+        .InitializeState(WorkflowExpr.Object(
+            WorkflowExpr.Prop("policyId", WorkflowExpr.Path("start.policyId"))))
+        .StartWith(BuildFlow)
+        .Build();
+
+    public IReadOnlyCollection<WorkflowTaskDescriptor> Tasks => Spec.TaskDescriptors;
+
+    private static void BuildFlow(WorkflowFlowBuilder<PolicyValidationRequest> flow)
+    {
+        flow
+            // --- Example 1: Simple call with shorthand error handling ---
+            .Call(
+                "Validate Policy",                                // step name
+                Address.LegacyRabbit("pas_policy_validate"),      // transport address
+                WorkflowExpr.Object(                              // payload (expression-based)
+                    WorkflowExpr.Prop("policyId", WorkflowExpr.Path("state.policyId"))),
+                WorkflowHandledBranchAction.Complete,             // on failure: complete workflow
+                WorkflowHandledBranchAction.Complete)             // on timeout: complete workflow
+
+            // --- Example 2: Call with typed response stored in state ---
+            .Call<object>(
+                "Load Policy Info",
+                Address.LegacyRabbit("pas_get_policy_product_info"),
+                WorkflowExpr.Object(
+                    WorkflowExpr.Prop("policyId", WorkflowExpr.Path("state.policyId"))),
+                WorkflowHandledBranchAction.Complete,
+                WorkflowHandledBranchAction.Complete,
+                resultKey: "policyInfo")                          // store response as "policyInfo"
+
+            // Use the result to set state values
+            .SetIfHasValue("productCode",
+                WorkflowExpr.Func("upper", WorkflowExpr.Path("result.policyInfo.productCode")))
+            .SetIfHasValue("lob",
+                WorkflowExpr.Path("result.policyInfo.lob"))
+
+            // --- Example 3: Call with custom failure/timeout branches ---
+            .Call(
+                "Calculate Premium",
+                Address.LegacyRabbit("pas_premium_calculate_for_object",
+                    SerdicaLegacyRabbitMode.MicroserviceConsumer),
+                WorkflowExpr.Object(
+                    WorkflowExpr.Prop("policyId", WorkflowExpr.Path("state.policyId"))),
+                whenFailure: fail => fail
+                    .Set("calculationFailed", WorkflowExpr.Bool(true))
+                    .Complete(),
+                whenTimeout: timeout => timeout
+                    .Set("calculationTimedOut", WorkflowExpr.Bool(true))
+                    .Complete(),
+                timeoutSeconds: 120)                              // per-step timeout: 2 minutes
+
+            // --- Example 4: HTTP transport ---
+            // .Call("Notify External",
+            //     Address.Http("authority", "/api/v1/notifications", "POST"),
+            //     WorkflowExpr.Object(
+            //         WorkflowExpr.Prop("message", WorkflowExpr.String("Policy validated"))),
+            //     WorkflowHandledBranchAction.Complete,
+            //     WorkflowHandledBranchAction.Complete)
+
+            .Complete();
+    }
+}