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/06-advanced-patterns/json/advanced-patterns.definition.json

@@ -0,0 +1,127 @@
+{
+  "schemaVersion": "serdica.workflow.definition/v1",
+  "workflowName": "AdvancedPatterns",
+  "workflowVersion": "1.0.0",
+  "displayName": "Advanced Patterns Example",
+  "workflowRoles": [],
+  "start": {
+    "initializeStateExpression": {
+      "$type": "object",
+      "properties": [
+        { "name": "policyId", "expression": { "$type": "path", "path": "start.policyId" } },
+        { "name": "retryAttempt", "expression": { "$type": "number", "value": 0 } },
+        { "name": "integrationFailed", "expression": { "$type": "boolean", "value": false } }
+      ]
+    },
+    "sequence": {
+      "steps": [
+        {
+          "$type": "fork",
+          "stepName": "Process Documents and Notify",
+          "branches": [
+            {
+              "steps": [
+                {
+                  "$type": "call-transport",
+                  "stepName": "Generate PDF",
+                  "invocation": {
+                    "address": { "$type": "legacy-rabbit", "command": "pas_pdf_generate", "mode": "Envelope" },
+                    "payloadExpression": {
+                      "$type": "object",
+                      "properties": [
+                        { "name": "policyId", "expression": { "$type": "path", "path": "state.policyId" } }
+                      ]
+                    }
+                  },
+                  "whenFailure": { "steps": [{ "$type": "complete" }] },
+                  "whenTimeout": { "steps": [{ "$type": "complete" }] }
+                }
+              ]
+            },
+            {
+              "steps": [
+                {
+                  "$type": "call-transport",
+                  "stepName": "Send Email",
+                  "invocation": {
+                    "address": { "$type": "legacy-rabbit", "command": "notifications_send_email", "mode": "MicroserviceConsumer" },
+                    "payloadExpression": {
+                      "$type": "object",
+                      "properties": [
+                        { "name": "to", "expression": { "$type": "string", "value": "agent@company.com" } },
+                        { "name": "subject", "expression": { "$type": "string", "value": "Policy processed" } }
+                      ]
+                    }
+                  },
+                  "whenFailure": { "steps": [{ "$type": "complete" }] },
+                  "whenTimeout": { "steps": [{ "$type": "complete" }] }
+                }
+              ]
+            }
+          ]
+        },
+        {
+          "$type": "repeat",
+          "stepName": "Retry Integration",
+          "maxIterationsExpression": { "$type": "number", "value": 3 },
+          "iterationStateKey": "retryAttempt",
+          "continueWhileExpression": {
+            "$type": "binary", "operator": "or",
+            "left": {
+              "$type": "binary", "operator": "eq",
+              "left": { "$type": "path", "path": "state.retryAttempt" },
+              "right": { "$type": "number", "value": 0 }
+            },
+            "right": { "$type": "path", "path": "state.integrationFailed" }
+          },
+          "body": {
+            "steps": [
+              { "$type": "set-state", "stateKey": "integrationFailed", "valueExpression": { "$type": "boolean", "value": false } },
+              {
+                "$type": "call-transport",
+                "stepName": "Transfer Policy",
+                "invocation": {
+                  "address": { "$type": "http", "target": "integration", "path": "/api/transfer", "method": "POST" },
+                  "payloadExpression": {
+                    "$type": "object",
+                    "properties": [
+                      { "name": "policyId", "expression": { "$type": "path", "path": "state.policyId" } }
+                    ]
+                  }
+                },
+                "whenFailure": {
+                  "steps": [
+                    { "$type": "set-state", "stateKey": "integrationFailed", "valueExpression": { "$type": "boolean", "value": true } },
+                    { "$type": "timer", "stepName": "Backoff", "delayExpression": { "$type": "string", "value": "00:05:00" } }
+                  ]
+                },
+                "whenTimeout": {
+                  "steps": [
+                    { "$type": "set-state", "stateKey": "integrationFailed", "valueExpression": { "$type": "boolean", "value": true } }
+                  ]
+                }
+              }
+            ]
+          }
+        },
+        {
+          "$type": "external-signal",
+          "stepName": "Wait for Document Upload",
+          "signalNameExpression": { "$type": "string", "value": "documents-ready" },
+          "resultKey": "uploadedDocs"
+        },
+        {
+          "$type": "set-state",
+          "stateKey": "documentCount",
+          "valueExpression": {
+            "$type": "function",
+            "functionName": "length",
+            "arguments": [{ "$type": "path", "path": "result.uploadedDocs.fileIds" }]
+          }
+        },
+        { "$type": "complete" }
+      ]
+    }
+  },
+  "tasks": []
+}