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/07-shared-helpers/README.md

@@ -0,0 +1,24 @@
+# Tutorial 7: Shared Support Helpers
+
+When building many workflows for the same domain (e.g., 50+ policy change workflows), extract reusable components into a support helper class.
+
+## What to Extract
+
+| Component | Example |
+|-----------|---------|
+| **Address constants** | `LegacyRabbitAddress`, `HttpAddress` — centralized routing |
+| **Workflow references** | `WorkflowReference` — for SubWorkflow/ContinueWith targets |
+| **Payload builders** | Static methods returning `WorkflowExpressionDefinition` |
+| **State initializers** | Base state + override pattern |
+| **Flow extensions** | Extension methods on `WorkflowFlowBuilder<T>` for common sequences |
+
+## C#-Only Tutorial
+
+This tutorial has no JSON equivalent — it covers C# code organization patterns.
+
+- [C# Example](csharp/)
+
+## Next
+
+[Tutorial 8: Expressions](../08-expressions/) — path navigation, functions, and operators.
+