feat(boxing): handle multi-code paichan switches

lib/services/boxing_context.dart

@@ -0,0 +1,38 @@
+enum PaichanContextChange { firstScan, samePaichan, switchedPaichan }
+
+class PaichanContextDecision {
+  final PaichanContextChange change;
+  final int? boxNoToApply;
+
+  const PaichanContextDecision({required this.change, this.boxNoToApply});
+
+  bool get isSwitched => change == PaichanContextChange.switchedPaichan;
+}
+
+PaichanContextDecision decideMultiCodePaichanContext({
+  required String? lastPaichanNo,
+  required String? currentPaichanNo,
+  required String currentBoxNoText,
+  required int maxBoxNo,
+}) {
+  final recommendedBoxNo = maxBoxNo + 1;
+  if (currentPaichanNo == null || currentPaichanNo.isEmpty) {
+    return const PaichanContextDecision(change: PaichanContextChange.firstScan);
+  }
+  if (lastPaichanNo == null || lastPaichanNo.isEmpty) {
+    return PaichanContextDecision(
+      change: PaichanContextChange.firstScan,
+      boxNoToApply: currentBoxNoText.trim().isEmpty ? recommendedBoxNo : null,
+    );
+  }
+  if (lastPaichanNo == currentPaichanNo) {
+    return PaichanContextDecision(
+      change: PaichanContextChange.samePaichan,
+      boxNoToApply: currentBoxNoText.trim().isEmpty ? recommendedBoxNo : null,
+    );
+  }
+  return PaichanContextDecision(
+    change: PaichanContextChange.switchedPaichan,
+    boxNoToApply: recommendedBoxNo,
+  );
+}

lib/services/feedback_service.dart

@@ -11,6 +11,7 @@ enum FeedbackEvent {
   networkError, // Network error         → failure sound + short vibrate
   duplicateBoxNo, // Duplicate box no     → failure sound + short vibrate
   modeSwitch, // Mode toggle           → beep only
+  paichanSwitch, // Paichan changed      → beep + short vibrate
 }
 
 /// Centralized feedback dispatcher.
@@ -56,6 +57,10 @@ class FeedbackService {
 
       case FeedbackEvent.modeSwitch:
         _soundService.playBeep();
+
+      case FeedbackEvent.paichanSwitch:
+        _soundService.playBeep();
+        _vibrateShort();
     }
   }