Changes:
- NoteEditorViewModel.kt: Add calculateInsertIndexForNewItem() helper method
- NoteEditorViewModel.kt: Rewrite addChecklistItemAtEnd() to insert before
first checked item (MANUAL/UNCHECKED_FIRST) instead of appending at list end
- NoteEditorViewModel.kt: Add cross-boundary guard to addChecklistItemAfter()
preventing new unchecked items from being inserted inside checked section
- ChecklistSortingTest.kt: Add ChecklistSortOption import
- ChecklistSortingTest.kt: Add 10 IMPL_15 unit tests covering all sort modes,
edge cases (empty list, all checked, no checked), and position stability
Root cause: addChecklistItemAtEnd() appended new unchecked items at the end
of the flat list, after checked items. The UI splits items by count
(subList(0, uncheckedCount)), not by isChecked state — causing checked items
to appear above the separator and new items below it.
Fix: Insert new items at the semantically correct position per sort mode.
MANUAL/UNCHECKED_FIRST: before first checked item (above separator).
All other modes: at list end (no separator visible, no visual issue).
All 19 unit tests pass (9 existing + 10 new). No UI changes required.
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