fix(highlight): retry highlight computation when Normal has no background

Problem: if `Normal.bg` is nil when `compute_highlight_groups` first runs
(colorscheme not yet loaded), the neutral fallbacks are baked in and never
corrected — the `ColorScheme` autocmd only fires for explicit `:colorscheme`
calls, which may come too late for some plugin manager configurations.

Solution: when `Normal.bg` is absent, schedule a single deferred retry via
`vim.schedule` that recomputes all highlight groups and invalidates attached
buffer caches once the event loop settles. Also document the load-order
requirement and the retry behaviour in the highlight groups section of the
vimdoc.

doc/diffs.nvim.txt

@@ -758,6 +758,13 @@ character-level groups blend at 60% for more contrast. Line-number groups
 combine both: background from the line-level blend, foreground from the
 character-level blend.
 
+Note: highlight groups are computed when the first diff buffer is attached,
+so your colorscheme must be loaded before any diff buffer opens. If `Normal`
+has no background at that point (colorscheme loaded too late, or transparent
+terminal), diffs.nvim schedules one deferred retry via |vim.schedule()| to
+recompute once the event loop settles. To avoid relying on this, ensure your
+colorscheme plugin has `priority = 1000` and `lazy = false`.
+
 Fugitive unified diff highlights: ~
                                                                   *DiffsAdd*
     DiffsAdd            Background for `+` lines. Derived by blending