Selection/Placement highlight in assembler.

It can be difficult to see which voxel is selected in the assembler. While the system picks the voxel under the mouse pointer with precision, this isn’t always obvious.

Selection highlight: Add a shader to show the voxel about to be selected, reducing guesswork and improving clarity.

Placement highlight: Apply the same shader to the building floor or target voxel once a module is selected, making placement clearer and reducing errors.

These changes would make the assembler more transparent and responsive, following a similar behaviour to the stamper visually indicator.