Why are almost all my Windows 10 icons showing as generic placeholders?

Why did almost every icon on my Windows 10 Pro PC suddenly turn into a generic placeholder, including desktop icons, File Explorer icons, most app icons, and even the icons shown next to processes in Task Manager? The files themselves are fine—Explorer still shows correct filenames, folders, and drives—so this looks like an icon-rendering problem rather than anything wrong with the files. Curiously, a small number of icons still display normally.

I've already restarted Explorer multiple times, rebuilt the icon cache, checked the usual registry associations for exefile, Directory, Folder, Drive, and .lnk, confirmed HideIcons is 0, and even removed the per-user Shell Icons key, but none of that helped. I also noticed that the expected iconcache_*.db files weren't present in the Explorer icon-cache folder. What stood out most is that SFC reported C:\Windows\SystemResources\imageres.dll.mun didn't match the expected hash and tried to repair it from the component store, and I've since started DISM /Online /Cleanup-Image /RestoreHealth. I do use 7TSP with a Big Sur-style icon and theme pack, but the icons were working fine before this started. Is this a corrupted Windows resource, an icon cache problem, a registry/icon-handler issue, or something caused by the customization tool—and what's the best way to tell which one it is without reinstalling Windows?