R1:665529ab60a1

Restore coverage reporting to Diffusion browse UI

Summary:
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.

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

{F5525542}

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