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/04-human-tasks/README.md

@@ -0,0 +1,34 @@
+# Tutorial 4: Human Tasks with OnComplete Flows
+
+The approve/reject pattern — the most common human task flow in insurance workflows.
+
+## Concepts Introduced
+
+- `WorkflowHumanTask.For<T>()` — define a task with name, type, route, and roles
+- `.WithPayload()` — data sent to the UI when the task is displayed
+- `.WithTimeout(seconds)` — optional deadline for the task
+- `.WithRoles()` — restrict which roles can interact with this task
+- `.OnComplete(flow => ...)` — sequence executed after user completes the task
+- `.ActivateTask()` — pause workflow and wait for user action
+- `.AddTask()` — register a task in the workflow spec (separate from activation)
+- Re-activation — send the user back to the same task on validation failure
+
+## Approve/Reject Pattern
+
+1. Workflow starts, runs some service tasks
+2. `.ActivateTask("Approve")` — workflow pauses
+3. User sees the task in their inbox, assigns it, submits an answer
+4. `.OnComplete` checks `payload.answer`:
+   - `"approve"` — run confirmation operations, convert to policy
+   - `"reject"` — cancel the application
+5. If operations fail, re-activate the same task for correction
+
+## Variants
+
+- [C# Fluent DSL](csharp/)
+- [Canonical JSON](json/)
+
+## Next
+
+[Tutorial 5: Sub-Workflows](../05-sub-workflows/) — inline vs fire-and-forget child workflows.
+