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/01-hello-world/README.md

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+# Tutorial 1: Hello World
+
+The simplest possible workflow: initialize state from a start request, activate a single human task, and complete the workflow when the task is done.
+
+## Concepts Introduced
+
+- `IDeclarativeWorkflow<T>` — the contract every workflow implements
+- `WorkflowSpec.For<T>()` — the builder entry point
+- `.InitializeState()` — transforms the start request into workflow state
+- `.StartWith(task)` — sets the first task to activate
+- `WorkflowHumanTask.For<T>()` — defines a human task
+- `.OnComplete(flow => flow.Complete())` — terminal step
+
+## What Happens at Runtime
+
+1. Client calls `StartWorkflowAsync` with `WorkflowName = "Greeting"` and payload `{ "customerName": "John" }`
+2. State initializes to `{ "customerName": "John" }`
+3. Task "Greet Customer" is created with status "Pending"
+4. A user assigns the task to themselves, then completes it
+5. `OnComplete` executes `.Complete()` — the workflow finishes
+
+## Variants
+
+- [C# Fluent DSL](csharp/)
+- [Canonical JSON](json/)
+
+## Next
+
+[Tutorial 2: Service Tasks](../02-service-tasks/) — call external services before or after human tasks.
+

docs/workflow/tutorials/01-hello-world/csharp/GreetingWorkflow.cs

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+using System.Collections.Generic;
+using System.Text.Json;
+
+using WorkflowEngine.Abstractions;
+using WorkflowEngine.Contracts;
+
+namespace WorkflowEngine.Tutorials;
+
+// Start request — defines the input contract for the workflow.
+public sealed class GreetingRequest
+{
+    public string CustomerName { get; set; } = string.Empty;
+}
+
+// Workflow definition — implements IDeclarativeWorkflow<TStartRequest>.
+public sealed class GreetingWorkflow : IDeclarativeWorkflow<GreetingRequest>
+{
+    // Identity: name + version uniquely identify the workflow definition.
+    public string WorkflowName => "Greeting";
+    public string WorkflowVersion => "1.0.0";
+    public string DisplayName => "Customer Greeting";
+
+    // Roles: which user roles can see and interact with this workflow's tasks.
+    public IReadOnlyCollection<string> WorkflowRoles => ["DBA", "UR_AGENT"];
+
+    // Spec: the workflow specification built via the fluent DSL.
+    public WorkflowSpec<GreetingRequest> Spec { get; } = WorkflowSpec
+        .For<GreetingRequest>()
+
+        // InitializeState: transform the start request into the workflow's mutable state.
+        // State is a Dictionary<string, JsonElement> — all values are JSON-serialized.
+        .InitializeState(request => new Dictionary<string, JsonElement>
+        {
+            ["customerName"] = JsonSerializer.SerializeToElement(request.CustomerName),
+        })
+
+        // StartWith: register and activate this task as the first step.
+        .StartWith(greetTask)
+        .Build();
+
+    // Tasks: expose task descriptors for the registration catalog.
+    public IReadOnlyCollection<WorkflowTaskDescriptor> Tasks => Spec.TaskDescriptors;
+
+    // Task definition: defines name, type (UI component), route (navigation), and behavior.
+    private static readonly WorkflowHumanTaskDefinition<GreetingRequest> greetTask =
+        WorkflowHumanTask.For<GreetingRequest>(
+                taskName: "Greet Customer",          // unique name within this workflow
+                taskType: "GreetCustomerTask",        // UI component identifier
+                route: "customers/greet")             // navigation route
+            .WithPayload(context => new Dictionary<string, JsonElement>
+            {
+                // Pass state values to the task's UI payload.
+                ["customerName"] = context.StateValues
+                    .GetRequired<string>("customerName").AsJsonElement(),
+            })
+            // OnComplete: what happens after the user completes this task.
+            .OnComplete(flow => flow.Complete());   // simply end the workflow
+}

docs/workflow/tutorials/01-hello-world/json/greeting-workflow.definition.json

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+{
+  "schemaVersion": "serdica.workflow.definition/v1",
+  "workflowName": "Greeting",
+  "workflowVersion": "1.0.0",
+  "displayName": "Customer Greeting",
+
+  "startRequest": {
+    "contractName": "GreetingRequest",
+    "allowAdditionalProperties": true
+  },
+
+  "workflowRoles": ["DBA", "UR_AGENT"],
+
+  "start": {
+    "initializeStateExpression": {
+      "$type": "object",
+      "properties": [
+        {
+          "name": "customerName",
+          "expression": { "$type": "path", "path": "start.customerName" }
+        }
+      ]
+    },
+    "sequence": {
+      "steps": [
+        {
+          "$type": "activate-task",
+          "taskName": "Greet Customer"
+        }
+      ]
+    }
+  },
+
+  "tasks": [
+    {
+      "taskName": "Greet Customer",
+      "taskType": "GreetCustomerTask",
+      "routeExpression": { "$type": "string", "value": "customers/greet" },
+      "taskRoles": [],
+      "payloadExpression": {
+        "$type": "object",
+        "properties": [
+          {
+            "name": "customerName",
+            "expression": { "$type": "path", "path": "state.customerName" }
+          }
+        ]
+      },
+      "onCompleteSequence": {
+        "steps": [
+          { "$type": "complete" }
+        ]
+      }
+    }
+  ]
+}