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,
+  );
+}