feat(boxing): handle multi-code paichan switches

test/services/boxing_context_test.dart

@@ -0,0 +1,60 @@
+import 'package:flutter_test/flutter_test.dart';
+import 'package:pad_scanner/services/boxing_context.dart';
+
+void main() {
+  group('decideMultiCodePaichanContext', () {
+    test('first scan records context and fills recommended box if empty', () {
+      final decision = decideMultiCodePaichanContext(
+        lastPaichanNo: null,
+        currentPaichanNo: 'W00009',
+        currentBoxNoText: '',
+        maxBoxNo: 3,
+      );
+
+      expect(decision.change, PaichanContextChange.firstScan);
+      expect(decision.boxNoToApply, 4);
+      expect(decision.isSwitched, isFalse);
+    });
+
+    test('same paichan keeps current box number', () {
+      final decision = decideMultiCodePaichanContext(
+        lastPaichanNo: 'W00009',
+        currentPaichanNo: 'W00009',
+        currentBoxNoText: '8',
+        maxBoxNo: 3,
+      );
+
+      expect(decision.change, PaichanContextChange.samePaichan);
+      expect(decision.boxNoToApply, isNull);
+      expect(decision.isSwitched, isFalse);
+    });
+
+    test('switched paichan resets to new paichan max box plus one', () {
+      final decision = decideMultiCodePaichanContext(
+        lastPaichanNo: 'W00009',
+        currentPaichanNo: 'W00010',
+        currentBoxNoText: '8',
+        maxBoxNo: 1,
+      );
+
+      expect(decision.change, PaichanContextChange.switchedPaichan);
+      expect(decision.boxNoToApply, 2);
+      expect(decision.isSwitched, isTrue);
+    });
+
+    test(
+      'same paichan defensively fills recommended box when box is empty',
+      () {
+        final decision = decideMultiCodePaichanContext(
+          lastPaichanNo: 'W00009',
+          currentPaichanNo: 'W00009',
+          currentBoxNoText: '',
+          maxBoxNo: 4,
+        );
+
+        expect(decision.change, PaichanContextChange.samePaichan);
+        expect(decision.boxNoToApply, 5);
+      },
+    );
+  });
+}