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/Workflow/StellaOps.Workflow.WebService/appsettings.json

@@ -0,0 +1,33 @@
+{
+  "Logging": {
+    "LogLevel": {
+      "Default": "Information",
+      "Microsoft.AspNetCore": "Warning"
+    }
+  },
+  "Serilog": {
+    "MinimumLevel": {
+      "Default": "Information",
+      "Override": {
+        "Microsoft.AspNetCore": "Warning",
+        "System": "Warning"
+      }
+    },
+    "WriteTo": [
+      { "Name": "Console" }
+    ]
+  },
+  "WorkflowBackend": {
+    "Provider": "Mongo",
+    "Mongo": {
+      "ConnectionStringName": "WorkflowMongo",
+      "DatabaseName": "stellaops_workflow"
+    }
+  },
+  "WorkflowSignalDriver": {
+    "Provider": "Native"
+  },
+  "ConnectionStrings": {
+    "WorkflowMongo": "mongodb://localhost:27017"
+  }
+}