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/02-service-tasks/json/service-task-workflow.definition.json

@@ -0,0 +1,89 @@
+{
+  "schemaVersion": "serdica.workflow.definition/v1",
+  "workflowName": "PolicyValidation",
+  "workflowVersion": "1.0.0",
+  "displayName": "Policy Validation",
+  "workflowRoles": ["DBA"],
+
+  "start": {
+    "initializeStateExpression": {
+      "$type": "object",
+      "properties": [
+        { "name": "policyId", "expression": { "$type": "path", "path": "start.policyId" } }
+      ]
+    },
+    "sequence": {
+      "steps": [
+        {
+          "$type": "call-transport",
+          "stepName": "Validate Policy",
+          "invocation": {
+            "address": { "$type": "legacy-rabbit", "command": "pas_policy_validate", "mode": "Envelope" },
+            "payloadExpression": {
+              "$type": "object",
+              "properties": [
+                { "name": "policyId", "expression": { "$type": "path", "path": "state.policyId" } }
+              ]
+            }
+          },
+          "whenFailure": { "steps": [{ "$type": "complete" }] },
+          "whenTimeout": { "steps": [{ "$type": "complete" }] }
+        },
+        {
+          "$type": "call-transport",
+          "stepName": "Load Policy Info",
+          "resultKey": "policyInfo",
+          "invocation": {
+            "address": { "$type": "legacy-rabbit", "command": "pas_get_policy_product_info", "mode": "Envelope" },
+            "payloadExpression": {
+              "$type": "object",
+              "properties": [
+                { "name": "policyId", "expression": { "$type": "path", "path": "state.policyId" } }
+              ]
+            }
+          },
+          "whenFailure": { "steps": [{ "$type": "complete" }] },
+          "whenTimeout": { "steps": [{ "$type": "complete" }] }
+        },
+        {
+          "$type": "set-state",
+          "stateKey": "productCode",
+          "valueExpression": {
+            "$type": "function",
+            "functionName": "upper",
+            "arguments": [{ "$type": "path", "path": "result.policyInfo.productCode" }]
+          },
+          "onlyIfPresent": true
+        },
+        {
+          "$type": "call-transport",
+          "stepName": "Calculate Premium",
+          "timeoutSeconds": 120,
+          "invocation": {
+            "address": { "$type": "legacy-rabbit", "command": "pas_premium_calculate_for_object", "mode": "MicroserviceConsumer" },
+            "payloadExpression": {
+              "$type": "object",
+              "properties": [
+                { "name": "policyId", "expression": { "$type": "path", "path": "state.policyId" } }
+              ]
+            }
+          },
+          "whenFailure": {
+            "steps": [
+              { "$type": "set-state", "stateKey": "calculationFailed", "valueExpression": { "$type": "boolean", "value": true } },
+              { "$type": "complete" }
+            ]
+          },
+          "whenTimeout": {
+            "steps": [
+              { "$type": "set-state", "stateKey": "calculationTimedOut", "valueExpression": { "$type": "boolean", "value": true } },
+              { "$type": "complete" }
+            ]
+          }
+        },
+        { "$type": "complete" }
+      ]
+    }
+  },
+  "tasks": []
+}