fix(highlight): use theme-agnostic fallbacks and retry when Normal has no background (#153)

## 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.

doc/diffs.nvim.txt

@@ -65,6 +65,9 @@ Install with lazy.nvim: >lua
 The plugin works automatically with no configuration required. For
 customization, see |diffs-config|.
 
+NOTE: Load your colorscheme before `diffs.nvim`. For example, with lazy.nvim,
+set `priority = 1000` and `lazy = false` on your colorscheme plugin.
+
 ==============================================================================
 CONFIGURATION                                                   *diffs-config*
 
@@ -758,6 +761,7 @@ 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.
 
+
 Fugitive unified diff highlights: ~
                                                                   *DiffsAdd*
     DiffsAdd            Background for `+` lines. Derived by blending