refactor: split boxing page into part files for scan, submit, ops and status

Extract large methods from _BoxingPageState into 6 part files:
- boxing_scan_part.dart: scan handling and auto-scan processing
- boxing_submit_part.dart: submit/update/delete API calls
- boxing_box_mutation_part.dart: box item mutations
- boxing_single_ops_part.dart: single-code boxing operations
- boxing_multi_ops_part.dart: multi-code boxing operations
- boxing_status_part.dart: status bar management

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

lib/pages/boxing/parts/boxing_box_mutation_part.dart

@@ -0,0 +1,59 @@
+part of '../../boxing_page.dart';
+
+extension _BoxingBoxMutationPart on _BoxingPageState {
+  void _replaceExistingBoxItem(
+    CurrentZongpaiBoxData editing,
+    int boxNo,
+    int quantity,
+  ) {
+    final result = box_mutations.replaceExistingBoxItem(
+      existingBoxes: _existingBoxes,
+      editing: editing,
+      boxNo: boxNo,
+      quantity: quantity,
+      zongpaiNo: _zongpaiNo!,
+      workOrderNo: _workOrderNo,
+      totalQuantity: _erpQuantity,
+    );
+    _existingBoxes = result.boxes;
+    _maxBoxNo = result.maxBoxNo;
+  }
+
+  void _addExistingBoxItem(int boxNo, int quantity, int? boxItemId) {
+    final result = box_mutations.addExistingBoxItem(
+      existingBoxes: _existingBoxes,
+      boxNo: boxNo,
+      quantity: quantity,
+      boxItemId: boxItemId,
+      zongpaiNo: _zongpaiNo!,
+      workOrderNo: _workOrderNo,
+      totalQuantity: _erpQuantity,
+    );
+    _existingBoxes = result.boxes;
+    _maxBoxNo = result.maxBoxNo;
+  }
+
+  void _replaceExistingPackedItem(
+    ManyToOnePackedItem item,
+    int boxNo,
+    int quantity,
+  ) {
+    final result = box_mutations.replaceExistingPackedItem(
+      existingBoxes: _existingBoxes,
+      item: item,
+      boxNo: boxNo,
+      quantity: quantity,
+    );
+    _existingBoxes = result.boxes;
+    _maxBoxNo = result.maxBoxNo;
+  }
+
+  void _removeExistingBoxItem(int boxItemId) {
+    final result = box_mutations.removeExistingBoxItem(
+      existingBoxes: _existingBoxes,
+      boxItemId: boxItemId,
+    );
+    _existingBoxes = result.boxes;
+    _maxBoxNo = result.maxBoxNo;
+  }
+}