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/09-testing/README.md

@@ -0,0 +1,29 @@
+# Tutorial 9: Testing Your Workflow
+
+Write unit tests for workflows using `RecordingSerdicaLegacyRabbitTransport` and `TechnicalStyleWorkflowTestHelpers`.
+
+## Test Setup Pattern
+
+1. Create a recording transport with pre-configured responses
+2. Build a test service provider via `TechnicalStyleWorkflowTestHelpers.CreateServiceProvider`
+3. Resolve `WorkflowRuntimeService` from DI
+4. Call `StartWorkflowAsync` with test payload
+5. Assert: tasks created, transport calls made, state values correct
+6. Optionally complete tasks and verify downstream behavior
+
+## What to Test
+
+| Scenario | Approach |
+|----------|----------|
+| Workflow starts correctly | Assert single open task after start |
+| Service calls made in order | `transport.Invocations.Select(x => x.Command).Should().Equal(...)` |
+| Rejection flow | Complete task with `"answer": "reject"`, verify cancel call |
+| Approval flow | Complete with `"answer": "approve"`, verify conversion calls |
+| Operations failure re-opens task | Check same task re-appears after operations return `passed: false` |
+| Sub-workflow creates child tasks | Query tasks by child workflow name |
+| Business reference set | `startResponse.BusinessReference.Key.Should().Be(...)` |
+
+## C#-Only Tutorial
+
+- [C# Test Examples](csharp/)
+