Lock an item to a specific position

**Is your feature request related to a problem? Please describe.**
I'm interested in having a specific item always appear in a locked position within the masonry layout (e.g.: top-right column/cell).

**Describe the solution you'd like**
The item should be able to appear in a location position, no matter if I add or remove items & re-render the Masonry component.

**Describe alternatives you've considered**
I have not considered alternatives, though I'm going to see if this can be done with the `usePositioner` hook in its current form.

**Additional context**
N/A

Start by reading the usePositioner hook and the Masonry component to understand how item positions are assigned and preserved during re-rendering. Clarify the API and behavior for locking an item to a specified position, including how additions and removals should affect it; done means the item remains in that position across updates.

Written by the indexing model from the issue text.