feat(fugitive): add status buffer keymaps for unified diffs

Adds du/dU keymaps to fugitive's :Git status buffer for opening unified
diffs instead of side-by-side diffs:
- du opens horizontal split (mirrors dd)
- dU opens vertical split (mirrors dv)

Parses status buffer lines to extract filename and detect section
(staged/unstaged/untracked). For staged files, diffs index vs HEAD.
For unstaged files, diffs working tree vs index.

Configurable via vim.g.diffs.fugitive.horizontal/vertical (set to
false to disable).

lua/diffs/init.lua

@@ -12,11 +12,16 @@
 ---@field treesitter diffs.TreesitterConfig
 ---@field vim diffs.VimConfig
 
+---@class diffs.FugitiveConfig
+---@field horizontal string|false
+---@field vertical string|false
+
 ---@class diffs.Config
 ---@field debug boolean
 ---@field debounce_ms integer
 ---@field hide_prefix boolean
 ---@field highlights diffs.Highlights
+---@field fugitive diffs.FugitiveConfig
 
 ---@class diffs
 ---@field attach fun(bufnr?: integer)
@@ -78,6 +83,10 @@ local default_config = {
       max_lines = 200,
     },
   },
+  fugitive = {
+    horizontal = 'du',
+    vertical = 'dU',
+  },
 }
 
 ---@type diffs.Config
@@ -219,6 +228,25 @@ local function init()
     end
   end
 
+  if opts.fugitive then
+    vim.validate({
+      ['fugitive.horizontal'] = {
+        opts.fugitive.horizontal,
+        function(v)
+          return v == false or type(v) == 'string'
+        end,
+        'string or false',
+      },
+      ['fugitive.vertical'] = {
+        opts.fugitive.vertical,
+        function(v)
+          return v == false or type(v) == 'string'
+        end,
+        'string or false',
+      },
+    })
+  end
+
   if opts.debounce_ms and opts.debounce_ms < 0 then
     error('diffs: debounce_ms must be >= 0')
   end
@@ -354,4 +382,10 @@ function M.detach_diff()
   end
 end
 
+---@return diffs.FugitiveConfig
+function M.get_fugitive_config()
+  init()
+  return config.fugitive
+end
+
 return M