feat: expose iteration/iterationSize fields for diagram overlay

Replace synthetic wrapper node approach with direct iteration fields:
- ProcessorNode gains iteration (child's index) and iterationSize
  (container's total) fields, populated from ClickHouse flat records
- Frontend hooks detect iteration containers from iterationSize != null
  instead of scanning for wrapper processorTypes
- useExecutionOverlay filters children by iteration field instead of
  wrapper nodes, eliminating ITERATION_WRAPPER_TYPES entirely
- Cleaner data contract: API returns exactly what the DB stores

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

ui/src/api/schema.d.ts

@@ -1852,6 +1852,10 @@ export interface components {
                 [key: string]: string;
             };
             /** Format: int32 */
+            iteration: number;
+            /** Format: int32 */
+            iterationSize: number;
+            /** Format: int32 */
             loopIndex: number;
             /** Format: int32 */
             loopSize: number;