feat: Implement note positioning logic and tests for position conversion

lib/data/note_positioning.dart

@@ -0,0 +1,46 @@
+const int notePositionScale = 10;
+const int notePositionStep = 1000 * notePositionScale;
+const int notePositionRebalanceThreshold = 1;
+
+int toStoredNotePosition(double position) {
+  return (position * notePositionScale).round();
+}
+
+double fromStoredNotePosition(int storedPosition) {
+  return storedPosition / notePositionScale;
+}
+
+int nextTopNotePosition(Iterable<int> storedPositions) {
+  int? highestPosition;
+
+  for (final int position in storedPositions) {
+    if (highestPosition == null || position > highestPosition) {
+      highestPosition = position;
+    }
+  }
+
+  if (highestPosition == null) {
+    return 0;
+  }
+
+  return highestPosition + notePositionStep;
+}
+
+int? midpointNotePosition({
+  required int higherPosition,
+  required int lowerPosition,
+}) {
+  final int gap = higherPosition - lowerPosition;
+  if (gap <= notePositionRebalanceThreshold) {
+    return null;
+  }
+
+  return lowerPosition + (gap ~/ 2);
+}
+
+List<int> rebalanceNotePositions(int itemCount) {
+  return List<int>.generate(
+    itemCount,
+    (int index) => (itemCount - 1 - index) * notePositionStep,
+  );
+}