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/06-advanced-patterns/README.md

@@ -0,0 +1,22 @@
+# Tutorial 6: Advanced Patterns
+
+Fork (parallel branches), Repeat (retry loops), Timer (delays), and External Signal (wait for events).
+
+## Patterns
+
+| Pattern | Use When |
+|---------|----------|
+| **Fork** | Multiple independent operations that should run concurrently |
+| **Repeat** | Retry a service call with backoff, poll until condition met |
+| **Timer** | Delay between steps (backoff, scheduled processing) |
+| **External Signal** | Wait for an external event (document upload, approval from another system) |
+
+## Variants
+
+- [C# Fluent DSL](csharp/)
+- [Canonical JSON](json/)
+
+## Next
+
+[Tutorial 7: Shared Helpers](../07-shared-helpers/) — organizing reusable workflow components.
+