feat(boxing): handle multi-code paichan switches

lib/pages/boxing_page.dart

@@ -4,6 +4,7 @@ import 'package:pad_scanner/services/app_config_service.dart';
 import 'package:pad_scanner/services/scanner_service.dart';
 import 'package:pad_scanner/services/code_parser.dart';
 import 'package:pad_scanner/services/api_service.dart';
+import 'package:pad_scanner/services/boxing_context.dart';
 import 'package:pad_scanner/services/feedback_service.dart';
 import 'package:pad_scanner/pages/boxing_detail_page.dart';
 import 'package:pad_scanner/widgets/status_bar.dart';
@@ -101,6 +102,8 @@ class _BoxingPageState extends State<BoxingPage> {
   int? _editingPackedItemId;
   String _editingPackedQuantity = '';
   int? _deletingPackedItemId;
+  String? _lastScannedPaichanNo;
+  String? _paichanSwitchNotice;
 
   // 提交中
   bool _isSubmitting = false;
@@ -184,6 +187,8 @@ class _BoxingPageState extends State<BoxingPage> {
     _editingPackedItemId = null;
     _editingPackedQuantity = '';
     _deletingPackedItemId = null;
+    _lastScannedPaichanNo = null;
+    _paichanSwitchNotice = null;
     _isSubmitting = false;
     _lastBoxNo = null;
     _editingBox = null;
@@ -260,6 +265,9 @@ class _BoxingPageState extends State<BoxingPage> {
       _isDuplicateBoxNo = false;
       _editingBox = null;
       _statusOverrideText = null;
+      if (_mode == BoxingMode.singleCode) {
+        _paichanSwitchNotice = null;
+      }
 
       // 根据模式自动填充
       _applyAutoFill();
@@ -293,9 +301,27 @@ class _BoxingPageState extends State<BoxingPage> {
           }
         });
       case BoxingMode.multiCode:
-        if (_boxNoController.text.trim().isEmpty) {
-          _boxNoController.text = (_maxBoxNo + 1).toString();
+        final decision = decideMultiCodePaichanContext(
+          lastPaichanNo: _lastScannedPaichanNo,
+          currentPaichanNo: _paichanNo,
+          currentBoxNoText: _boxNoController.text,
+          maxBoxNo: _maxBoxNo,
+        );
+        if (decision.isSwitched) {
+          _manyToOnePackedItems.clear();
+          _editingPackedItemId = null;
+          _editingPackedQuantity = '';
+          _deletingPackedItemId = null;
+          _paichanSwitchNotice = '排产号已切换：${_paichanNo ?? "--"}';
+          _feedbackService.trigger(FeedbackEvent.paichanSwitch);
+        } else {
+          _paichanSwitchNotice = null;
         }
+        final boxNoToApply = decision.boxNoToApply;
+        if (boxNoToApply != null) {
+          _boxNoController.text = boxNoToApply.toString();
+        }
+        _lastScannedPaichanNo = _paichanNo;
         _quantityController.text = (_erpQuantity ?? 0).toString();
         _quantityController.selection = TextSelection(
           baseOffset: 0,
@@ -508,6 +534,7 @@ class _BoxingPageState extends State<BoxingPage> {
         }
 
       case BoxingMode.multiCode:
+        _lastScannedPaichanNo = _paichanNo;
         setState(() {
           if (boxItemId != null) {
             _manyToOnePackedItems.add(
@@ -643,6 +670,7 @@ class _BoxingPageState extends State<BoxingPage> {
   void _finishManyToOneBox() {
     final currentBoxNo = int.tryParse(_boxNoController.text.trim());
     setState(() {
+      _manyToOnePackedItems.clear();
       _editingPackedItemId = null;
       _editingPackedQuantity = '';
       _deletingPackedItemId = null;
@@ -913,6 +941,11 @@ class _BoxingPageState extends State<BoxingPage> {
       _statusText = '超出可装数量上限';
       return;
     }
+    if (_paichanSwitchNotice != null) {
+      _statusDot = StatusDotColor.blue;
+      _statusText = '排产号已切换，箱号已重置';
+      return;
+    }
     switch (_phase) {
       case _Phase.waiting:
         _statusDot = StatusDotColor.blue;
@@ -978,6 +1011,16 @@ class _BoxingPageState extends State<BoxingPage> {
       body: Column(
         children: [
           // 重复箱号警告条（保留，因为这是输入区关联的即时反馈）
+          if (_paichanSwitchNotice != null)
+            Container(
+              width: double.infinity,
+              padding: const EdgeInsets.symmetric(vertical: 8, horizontal: 16),
+              color: Colors.blue.shade50,
+              child: Text(
+                _paichanSwitchNotice!,
+                style: TextStyle(color: Colors.blue.shade900, fontSize: 13),
+              ),
+            ),
           if (_isDuplicateBoxNo && _phase == _Phase.scanned)
             Container(
               width: double.infinity,