feat: update OpenAPI spec and TypeScript types for execution overlay

Add iteration fields (loopIndex, loopSize, splitIndex, splitSize,
multicastIndex) to ProcessorNode schema. Add new endpoint path
/executions/{executionId}/processors/by-id/{processorId}/snapshot.
Remove stale diagramNodeId field that was dropped in V6 migration.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

ui/src/api/openapi.json

@@ -2359,6 +2359,61 @@
         }
       }
     },
+    "/executions/{executionId}/processors/by-id/{processorId}/snapshot": {
+      "get": {
+        "tags": [
+          "Detail"
+        ],
+        "summary": "Get exchange snapshot for a processor by processorId",
+        "operationId": "getProcessorSnapshotById",
+        "parameters": [
+          {
+            "name": "executionId",
+            "in": "path",
+            "required": true,
+            "schema": {
+              "type": "string"
+            }
+          },
+          {
+            "name": "processorId",
+            "in": "path",
+            "required": true,
+            "schema": {
+              "type": "string"
+            }
+          }
+        ],
+        "responses": {
+          "200": {
+            "description": "Snapshot data",
+            "content": {
+              "*/*": {
+                "schema": {
+                  "type": "object",
+                  "additionalProperties": {
+                    "type": "string"
+                  }
+                }
+              }
+            }
+          },
+          "404": {
+            "description": "Snapshot not found",
+            "content": {
+              "*/*": {
+                "schema": {
+                  "type": "object",
+                  "additionalProperties": {
+                    "type": "string"
+                  }
+                }
+              }
+            }
+          }
+        }
+      }
+    },
     "/diagrams": {
       "get": {
         "tags": [
@@ -4588,8 +4643,25 @@
             "type": "integer",
             "format": "int64"
           },
-          "diagramNodeId": {
-            "type": "string"
+          "loopIndex": {
+            "type": "integer",
+            "format": "int32"
+          },
+          "loopSize": {
+            "type": "integer",
+            "format": "int32"
+          },
+          "splitIndex": {
+            "type": "integer",
+            "format": "int32"
+          },
+          "splitSize": {
+            "type": "integer",
+            "format": "int32"
+          },
+          "multicastIndex": {
+            "type": "integer",
+            "format": "int32"
           },
           "errorMessage": {
             "type": "string"
@@ -4613,7 +4685,6 @@
         "required": [
           "attributes",
           "children",
-          "diagramNodeId",
           "durationMs",
           "endTime",
           "errorMessage",