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/08-expressions/README.md

@@ -0,0 +1,36 @@
+# Tutorial 8: Expressions
+
+The expression system enables declarative logic that compiles to portable canonical JSON. All expressions are evaluable at runtime without recompilation.
+
+## Path Navigation
+
+| Prefix | Source | Example |
+|--------|--------|---------|
+| `start.*` | Start request fields | `start.policyId` |
+| `state.*` | Mutable workflow state | `state.customerName` |
+| `payload.*` | Task completion payload | `payload.answer` |
+| `result.*` | Step result (by resultKey) | `result.productInfo.lob` |
+
+## Built-in Functions
+
+| Function | Description | Example |
+|----------|-------------|---------|
+| `coalesce` | First non-null | `coalesce(state.id, start.id, 0)` |
+| `concat` | String join | `concat("Policy #", state.policyNo)` |
+| `add` | Sum | `add(state.attempt, 1)` |
+| `if` | Conditional | `if(state.isVip, "VIP", "Standard")` |
+| `isNullOrWhiteSpace` | Null/empty check | `isNullOrWhiteSpace(state.name)` |
+| `length` | String/array length | `length(state.items)` |
+| `upper` | Uppercase | `upper(state.annexType)` |
+| `first` | First array element | `first(state.objects)` |
+| `mergeObjects` | Deep merge | `mergeObjects(state, payload)` |
+
+## Variants
+
+- [C# Expression Builder](csharp/)
+- [JSON Expression Format](json/)
+
+## Next
+
+[Tutorial 9: Testing](../09-testing/) — unit test setup with recording transports.
+