Depends on D19377. Ref T13125. Ref T13124. Ref T13105. Coverage reporting in Diffusion didn't initially survive the transition to Document Engine; restore it.

This adds some tentative/theoretical support for multiple columns of coverage, but no way to actually produce them in the UI. For now, the labels, codes, and colors are hard coded.

Added coverage with diffusion.updatecoverage, saw coverage in the UI:

Hovered over coverage, got labels and highlighting.

Double-checked labels for "N" (Not Executable) and "U" (Uncovered). See PHI577.

Faked some multi-column coverage, but you can't currently get this yourself today: