Surface-level Fill

I want a new argument or mode for the /fill command (e.g., /fill ... surface) that allows placing blocks only on the surface of the existing terrain within a defined area, instead of filling the entire 3D volume.

Currently, the /fill command always creates a solid geometric box (cuboid). When working with uneven terrain, like hills or stairs, it is impossible to "coat" or "overlay" the floor with a new material without either filling the air above it or manually executing dozens of commands for every single Y-level. This would be extremely helpful for spawn-proofing large areas (like Nether Fortresses) or landscaping, where you need the new blocks to follow the natural elevation of the world.

The command should check every X and Z coordinate in the selection, find the highest non-air block (the "floor"), and place the specified block at $Y+1$ (or replace the floor block itself).

Essentially, I want a "top-down projection" logic:

1. The command looks at the area from above.

2. It identifies the surface contour.

3. It applies the fill only to that specific surface layer, ignoring the empty space (air) between different heights.