Problem: fallback hex values for `Normal.bg`, `DiffAdd.bg`, etc. in
`compute_highlight_groups` were hardcoded to catppuccin mocha colors
(`0x1e1e2e`, `0x2e4a3a`, `0x4a2e3a`, etc.), producing visibly wrong
highlight groups for users on any other colorscheme when `Normal` has
no background (transparent terminals or early startup).
Solution: replace all hardcoded fallbacks with `vim.o.background`-aware
neutral values — dark mode gets near-black/grey, light mode gets
near-white/dark-grey — so the fallback path is theme-agnostic.
Barrett Ruth
904f141366
