Download the zip and do not unpack it. In Blender open Edit > Preferences > Add-ons, click the drop-down arrow in the top-right corner and choose Install from Disk, then pick the zip. Tick the checkbox next to LightmapForge. Press N in the 3D Viewport and open the Lightmap tab in the sidebar.

The same steps are in README.txt inside the zip; the full documentation is on the Documentation & FAQ tab of this page.

Blender 4.2 LTS and newer. Tested live on 4.2.9 and 5.2.0.

It draws your lightmap UVs into a grid the size of the lightmap and counts texels: coverage, texels claimed by two islands (light bleeding from one surface onto another), texels where one island folds onto itself, and whether the padding you set in texels is held. Audit Lightmap UVs does this on an existing layer without changing anything.

Smart UV Project is one of the candidates. Build Lightmap UVs also tries other angles, Lightmap Pack (below twenty thousand faces), a repack of your existing UVs, and on a second run the layout already in the layer. Each one is packed and measured, and the best measured layout wins per object. Because the current layout is a candidate, building again cannot make it worse.

No. It writes only the layer named in UV Layer and stores the chosen lightmap size on the object as a custom property (lightmap_resolution). Remove Lightmap UVs deletes only that layer and that property.

Yes. Mirror and Array give every copy the same UVs, so the exported mesh has two surfaces on the same texels. Objects with modifiers are measured both as they are and after the modifier stack, and the panel reports the second number when it is worse. Apply the modifier before building, or keep it out of the export.

No. The add-on contains no network code, uses no external packer and installs no Python packages. Pure Python, no compiled binaries.

Yes. The add-on is licensed under GPL-3.0; the full licence text ships inside the zip. The licence covers the add-on code; what you create with it is yours. The UVs it makes are yours too.