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>

src/__Libraries/StellaOps.ElkSharp/ElkModels.cs

@@ -0,0 +1,138 @@
+using System.Collections.Generic;
+using System.Threading;
+using System.Threading.Tasks;
+
+namespace StellaOps.ElkSharp;
+
+public enum ElkLayoutDirection
+{
+    TopToBottom = 0,
+    LeftToRight = 1,
+}
+
+public enum ElkLayoutEffort
+{
+    Draft = 0,
+    Balanced = 1,
+    Best = 2,
+}
+
+public sealed record ElkPort
+{
+    public required string Id { get; init; }
+    public string? Side { get; init; }
+    public double Width { get; init; } = 8;
+    public double Height { get; init; } = 8;
+}
+
+public sealed record ElkNode
+{
+    public required string Id { get; init; }
+    public required string Label { get; init; }
+    public required string Kind { get; init; }
+    public string? IconKey { get; init; }
+    public string? SemanticType { get; init; }
+    public string? SemanticKey { get; init; }
+    public string? Route { get; init; }
+    public string? TaskType { get; init; }
+    public string? ParentNodeId { get; init; }
+    public double Width { get; init; } = 160;
+    public double Height { get; init; } = 72;
+    public IReadOnlyCollection<ElkPort> Ports { get; init; } = [];
+}
+
+public sealed record ElkEdge
+{
+    public required string Id { get; init; }
+    public required string SourceNodeId { get; init; }
+    public required string TargetNodeId { get; init; }
+    public string? SourcePortId { get; init; }
+    public string? TargetPortId { get; init; }
+    public string? Kind { get; init; }
+    public string? Label { get; init; }
+}
+
+public sealed record ElkGraph
+{
+    public required string Id { get; init; }
+    public required IReadOnlyCollection<ElkNode> Nodes { get; init; }
+    public required IReadOnlyCollection<ElkEdge> Edges { get; init; }
+}
+
+public sealed record ElkLayoutOptions
+{
+    public ElkLayoutDirection Direction { get; init; } = ElkLayoutDirection.LeftToRight;
+    public double NodeSpacing { get; init; } = 40;
+    public double LayerSpacing { get; init; } = 60;
+    public ElkLayoutEffort Effort { get; init; } = ElkLayoutEffort.Best;
+    public int? OrderingIterations { get; init; }
+    public int? PlacementIterations { get; init; }
+}
+
+public sealed record ElkPoint
+{
+    public required double X { get; init; }
+    public required double Y { get; init; }
+}
+
+public sealed record ElkPositionedPort
+{
+    public required string Id { get; init; }
+    public string? Side { get; init; }
+    public double X { get; init; }
+    public double Y { get; init; }
+    public double Width { get; init; }
+    public double Height { get; init; }
+}
+
+public sealed record ElkPositionedNode
+{
+    public required string Id { get; init; }
+    public required string Label { get; init; }
+    public required string Kind { get; init; }
+    public string? IconKey { get; init; }
+    public string? SemanticType { get; init; }
+    public string? SemanticKey { get; init; }
+    public string? Route { get; init; }
+    public string? TaskType { get; init; }
+    public string? ParentNodeId { get; init; }
+    public double X { get; init; }
+    public double Y { get; init; }
+    public double Width { get; init; }
+    public double Height { get; init; }
+    public IReadOnlyCollection<ElkPositionedPort> Ports { get; init; } = [];
+}
+
+public sealed record ElkEdgeSection
+{
+    public required ElkPoint StartPoint { get; init; }
+    public required ElkPoint EndPoint { get; init; }
+    public IReadOnlyCollection<ElkPoint> BendPoints { get; init; } = [];
+}
+
+public sealed record ElkRoutedEdge
+{
+    public required string Id { get; init; }
+    public required string SourceNodeId { get; init; }
+    public required string TargetNodeId { get; init; }
+    public string? SourcePortId { get; init; }
+    public string? TargetPortId { get; init; }
+    public string? Kind { get; init; }
+    public string? Label { get; init; }
+    public IReadOnlyCollection<ElkEdgeSection> Sections { get; init; } = [];
+}
+
+public sealed record ElkLayoutResult
+{
+    public required string GraphId { get; init; }
+    public required IReadOnlyCollection<ElkPositionedNode> Nodes { get; init; }
+    public required IReadOnlyCollection<ElkRoutedEdge> Edges { get; init; }
+}
+
+public interface IElkLayoutEngine
+{
+    Task<ElkLayoutResult> LayoutAsync(
+        ElkGraph graph,
+        ElkLayoutOptions? options = null,
+        CancellationToken cancellationToken = default);
+}