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/03-decisions/README.md

@@ -0,0 +1,28 @@
+# Tutorial 3: Decisions
+
+Branch workflow logic based on conditions — state values, payload answers, or complex expressions.
+
+## Concepts Introduced
+
+- `.WhenExpression()` — branch on any boolean expression
+- `.WhenStateFlag()` — shorthand for checking a boolean state value
+- `.WhenPayloadEquals()` — shorthand for checking a task completion payload value
+- Nested decisions — decisions inside decisions for complex routing
+
+## Decision Types
+
+| Method | Use When |
+|--------|----------|
+| `WhenExpression` | Complex conditions (comparisons, boolean logic, function calls) |
+| `WhenStateFlag` | Checking a boolean state key against true/false |
+| `WhenPayloadEquals` | Checking a task completion answer (inside OnComplete) |
+
+## Variants
+
+- [C# Fluent DSL](csharp/)
+- [Canonical JSON](json/)
+
+## Next
+
+[Tutorial 4: Human Tasks](../04-human-tasks/) — approve/reject patterns with OnComplete flows.
+