## Problem
Highlight group fallbacks in `compute_highlight_groups` were hardcoded
to
catppuccin mocha colors, producing wrong results for any other
colorscheme
when `Normal.bg` is nil. This happens on transparent terminals or when
the
colorscheme loads after the first diff buffer opens.
## Solution
Replace hardcoded fallbacks with `vim.o.background`-aware neutral
values.
When `Normal.bg` is still absent after initial computation, schedule a
single
deferred retry via `vim.schedule` that recomputes and invalidates all
attached
buffer caches. Document the load-order requirement in the setup section.
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