haltu/muuri ? reverse-engineered prompt

Build me a JavaScript layout library like Muuri that lets me place items in a responsive grid and then drag, sort, filter, and animate them smoothly. I want it to work with normal HTML markup, support items of different sizes, and automatically rearrange everything when items are hidden, moved, or resized.

Please include a simple demo page so I can see it working right away, with a grid of cards that I can drag around, filter by a button click, and sort by a few different rules. It should feel fast and polished, and it should handle nested layouts if possible.

If you need to look up current browser behavior or docs online, feel free to do that. Make the code clean and easy to understand, and add the basic tests and build setup so I can run and tweak it locally.