feat: Implement note positioning logic and tests for position conversion

test/note_positioning_test.dart

@@ -0,0 +1,23 @@
+import 'package:flutter_test/flutter_test.dart';
+
+import 'package:notas/data/note_positioning.dart';
+
+void main() {
+  test('converts between stored and displayed positions', () {
+    expect(toStoredNotePosition(1500.5), 15005);
+    expect(fromStoredNotePosition(15005), 1500.5);
+  });
+
+  test('supports descending gaps and rebalance', () {
+    expect(nextTopNotePosition(<int>[0, 10000, 20000]), 30000);
+    expect(
+      midpointNotePosition(higherPosition: 20000, lowerPosition: 10000),
+      15000,
+    );
+    expect(
+      midpointNotePosition(higherPosition: 2, lowerPosition: 1),
+      isNull,
+    );
+    expect(rebalanceNotePositions(3), <int>[20000, 10000, 0]);
+  });
+}