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/02-service-tasks/README.md

@@ -0,0 +1,29 @@
+# Tutorial 2: Service Tasks
+
+Call external services (microservices, HTTP APIs, GraphQL, RabbitMQ) from within a workflow. Handle failures and timeouts gracefully.
+
+## Concepts Introduced
+
+- `.Call()` — invoke a transport with payload and optional response capture
+- Address types — `LegacyRabbit`, `Microservice`, `Http`, `Graphql`, `Rabbit`
+- `resultKey` — store the service response in workflow state
+- `whenFailure` / `whenTimeout` — recovery branches
+- `WorkflowHandledBranchAction.Complete` — shorthand for "complete on error"
+- `timeoutSeconds` — per-step timeout override (default: 1 hour)
+
+## Key Points
+
+- Each `Call` step executes synchronously within the workflow
+- The per-step timeout wraps the entire call including transport-level retries
+- Transport timeouts (30s default) control individual attempt duration
+- If no failure/timeout handler is defined, the error propagates and the signal pump retries
+
+## Variants
+
+- [C# Fluent DSL](csharp/)
+- [Canonical JSON](json/)
+
+## Next
+
+[Tutorial 3: Decisions](../03-decisions/) — branch workflow logic based on conditions.
+