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/03-decisions/csharp/DecisionWorkflow.cs

@@ -0,0 +1,72 @@
+using System.Collections.Generic;
+
+using WorkflowEngine.Abstractions;
+using WorkflowEngine.Contracts;
+
+namespace WorkflowEngine.Tutorials;
+
+public sealed class PolicyRoutingRequest
+{
+    public long PolicyId { get; set; }
+    public string AnnexType { get; set; } = string.Empty;
+    public bool PolicyExistsOnIPAL { get; set; }
+}
+
+public sealed class PolicyRoutingWorkflow : IDeclarativeWorkflow<PolicyRoutingRequest>
+{
+    public string WorkflowName => "PolicyRouting";
+    public string WorkflowVersion => "1.0.0";
+    public string DisplayName => "Policy Routing Example";
+    public IReadOnlyCollection<string> WorkflowRoles => ["DBA"];
+
+    public WorkflowSpec<PolicyRoutingRequest> Spec { get; } = WorkflowSpec
+        .For<PolicyRoutingRequest>()
+        .InitializeState(WorkflowExpr.Object(
+            WorkflowExpr.Prop("policyId", WorkflowExpr.Path("start.policyId")),
+            WorkflowExpr.Prop("annexType", WorkflowExpr.Path("start.annexType")),
+            WorkflowExpr.Prop("policyExistsOnIPAL",
+                WorkflowExpr.Func("coalesce",
+                    WorkflowExpr.Path("start.policyExistsOnIPAL"),
+                    WorkflowExpr.Bool(true)))))
+        .StartWith(BuildFlow)
+        .Build();
+
+    public IReadOnlyCollection<WorkflowTaskDescriptor> Tasks => Spec.TaskDescriptors;
+
+    private static void BuildFlow(WorkflowFlowBuilder<PolicyRoutingRequest> flow)
+    {
+        // --- Example 1: State flag decision (boolean shorthand) ---
+        flow.WhenStateFlag(
+            "policyExistsOnIPAL",             // state key to check
+            true,                              // expected value
+            "Policy exists on IPAL?",          // decision name (appears in diagram)
+            whenTrue: ipal => ipal
+
+                // --- Example 2: Expression decision ---
+                .WhenExpression(
+                    "Annex Type?",
+                    WorkflowExpr.Eq(
+                        WorkflowExpr.Func("upper", WorkflowExpr.Path("state.annexType")),
+                        WorkflowExpr.String("BENEF")),
+                    benefit => benefit
+                        .Set("route", WorkflowExpr.String("BENEFIT_PROCESSING"))
+                        .Complete(),
+
+                    // --- Example 3: Nested decision ---
+                    other => other.WhenExpression(
+                        "Is Equipment?",
+                        WorkflowExpr.Eq(
+                            WorkflowExpr.Func("upper", WorkflowExpr.Path("state.annexType")),
+                            WorkflowExpr.String("ADDEQ")),
+                        equipment => equipment
+                            .Set("route", WorkflowExpr.String("EQUIPMENT_PROCESSING"))
+                            .Complete(),
+                        cover => cover
+                            .Set("route", WorkflowExpr.String("COVER_CHANGE"))
+                            .Complete())),
+
+            whenElse: notIpal => notIpal
+                .Set("route", WorkflowExpr.String("INSIS_PROCESSING"))
+                .Complete());
+    }
+}