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/05-sub-workflows/README.md

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+# Tutorial 5: Sub-Workflows & Continuations
+
+Compose workflows by invoking child workflows — either inline (SubWorkflow) or fire-and-forget (ContinueWith).
+
+## SubWorkflow vs ContinueWith
+
+| Feature | `.SubWorkflow()` | `.ContinueWith()` |
+|---------|-----------------|-------------------|
+| Parent waits | Yes — resumes after child completes | No — parent completes immediately |
+| State flows back | Yes — child state merges into parent | No — child is independent |
+| Same instance | Yes — tasks appear under parent instance | No — new workflow instance |
+| Use when | Steps must complete before parent continues | Fire-and-forget, scheduled work |
+
+## Variants
+
+- [C# Fluent DSL](csharp/)
+- [Canonical JSON](json/)
+
+## Next
+
+[Tutorial 6: Advanced Patterns](../06-advanced-patterns/) — Fork, Repeat, Timer, External Signal.
+

docs/workflow/tutorials/05-sub-workflows/csharp/SubWorkflowExample.cs

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+using System.Collections.Generic;
+
+using WorkflowEngine.Abstractions;
+using WorkflowEngine.Contracts;
+
+namespace WorkflowEngine.Tutorials;
+
+public sealed class ParentWorkflow : IDeclarativeWorkflow<PolicyChangeWorkflowRequest>
+{
+    public string WorkflowName => "ParentWorkflow";
+    public string WorkflowVersion => "1.0.0";
+    public string DisplayName => "Parent Workflow Example";
+    public IReadOnlyCollection<string> WorkflowRoles => [];
+
+    public WorkflowSpec<PolicyChangeWorkflowRequest> Spec { get; } = WorkflowSpec
+        .For<PolicyChangeWorkflowRequest>()
+        .InitializeState(WorkflowExpr.Object(
+            WorkflowExpr.Prop("policyId", WorkflowExpr.Path("start.policyId"))))
+        .StartWith(BuildFlow)
+        .Build();
+
+    public IReadOnlyCollection<WorkflowTaskDescriptor> Tasks => Spec.TaskDescriptors;
+
+    private static void BuildFlow(WorkflowFlowBuilder<PolicyChangeWorkflowRequest> flow)
+    {
+        flow
+            .Call("Open For Change",
+                Address.LegacyRabbit("pas_annexprocessing_alterpolicy"),
+                WorkflowExpr.Object(
+                    WorkflowExpr.Prop("policyId", WorkflowExpr.Path("state.policyId"))),
+                WorkflowHandledBranchAction.Complete,
+                WorkflowHandledBranchAction.Complete)
+
+            // --- SubWorkflow: inline execution, parent waits ---
+            // The child workflow runs within this execution.
+            // Its tasks appear under the parent instance.
+            // State from the child merges back into the parent after completion.
+            .SubWorkflow(
+                "Review Policy Changes",
+                new WorkflowWorkflowInvocationDeclaration
+                {
+                    WorkflowName = "ReviewPolicyOpenForChange",
+                    PayloadExpression = WorkflowExpr.Object(
+                        WorkflowExpr.Prop("policyId", WorkflowExpr.Path("state.policyId")),
+                        WorkflowExpr.Prop("productCode", WorkflowExpr.Path("state.productCode"))),
+                })
+            // Execution resumes here after child completes.
+
+            // --- ContinueWith: fire-and-forget ---
+            // The parent workflow completes immediately.
+            // A new independent workflow instance is created via the signal bus.
+            .ContinueWith(
+                "Start Transfer Process",
+                new WorkflowWorkflowInvocationDeclaration
+                {
+                    WorkflowName = "TransferPolicy",
+                    PayloadExpression = WorkflowExpr.Path("state"),
+                });
+        // Parent is now complete. TransferPolicy runs independently.
+    }
+}

docs/workflow/tutorials/05-sub-workflows/json/sub-workflow.definition.json

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+{
+  "schemaVersion": "serdica.workflow.definition/v1",
+  "workflowName": "ParentWorkflow",
+  "workflowVersion": "1.0.0",
+  "displayName": "Parent Workflow Example",
+  "workflowRoles": [],
+  "start": {
+    "initializeStateExpression": {
+      "$type": "object",
+      "properties": [
+        { "name": "policyId", "expression": { "$type": "path", "path": "start.policyId" } }
+      ]
+    },
+    "sequence": {
+      "steps": [
+        {
+          "$type": "call-transport",
+          "stepName": "Open For Change",
+          "invocation": {
+            "address": { "$type": "legacy-rabbit", "command": "pas_annexprocessing_alterpolicy", "mode": "Envelope" },
+            "payloadExpression": {
+              "$type": "object",
+              "properties": [
+                { "name": "policyId", "expression": { "$type": "path", "path": "state.policyId" } }
+              ]
+            }
+          },
+          "whenFailure": { "steps": [{ "$type": "complete" }] },
+          "whenTimeout": { "steps": [{ "$type": "complete" }] }
+        },
+        {
+          "$type": "sub-workflow",
+          "stepName": "Review Policy Changes",
+          "invocation": {
+            "workflowName": "ReviewPolicyOpenForChange",
+            "payloadExpression": {
+              "$type": "object",
+              "properties": [
+                { "name": "policyId", "expression": { "$type": "path", "path": "state.policyId" } },
+                { "name": "productCode", "expression": { "$type": "path", "path": "state.productCode" } }
+              ]
+            }
+          }
+        },
+        {
+          "$type": "continue-with-workflow",
+          "stepName": "Start Transfer Process",
+          "invocation": {
+            "workflowName": "TransferPolicy",
+            "payloadExpression": { "$type": "path", "path": "state" }
+          }
+        }
+      ]
+    }
+  },
+  "tasks": []
+}