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/fugitive.lua

@@ -0,0 +1,145 @@
+local M = {}
+
+local commands = require('diffs.commands')
+local git = require('diffs.git')
+local dbg = require('diffs.log').dbg
+
+---@alias diffs.FugitiveSection 'staged' | 'unstaged' | 'untracked' | nil
+
+---@param bufnr integer
+---@param lnum integer
+---@return diffs.FugitiveSection
+function M.get_section_at_line(bufnr, lnum)
+  local lines = vim.api.nvim_buf_get_lines(bufnr, 0, lnum, false)
+
+  for i = #lines, 1, -1 do
+    local line = lines[i]
+    if line:match('^Staged ') then
+      return 'staged'
+    elseif line:match('^Unstaged ') then
+      return 'unstaged'
+    elseif line:match('^Untracked ') then
+      return 'untracked'
+    end
+  end
+
+  return nil
+end
+
+---@param line string
+---@return string?
+local function parse_file_line(line)
+  local renamed = line:match('^R[%s%d]*[^%s]+%s*->%s*(.+)$')
+  if renamed then
+    return vim.trim(renamed)
+  end
+
+  local filename = line:match('^[MADRCU?][MADRCU%s]*%s+(.+)$')
+  if filename then
+    return vim.trim(filename)
+  end
+
+  return nil
+end
+
+---@param bufnr integer
+---@param lnum integer
+---@return string?, diffs.FugitiveSection
+function M.get_file_at_line(bufnr, lnum)
+  local lines = vim.api.nvim_buf_get_lines(bufnr, 0, -1, false)
+  local current_line = lines[lnum]
+
+  if not current_line then
+    return nil, nil
+  end
+
+  local filename = parse_file_line(current_line)
+  if filename then
+    local section = M.get_section_at_line(bufnr, lnum)
+    return filename, section
+  end
+
+  local prefix = current_line:sub(1, 1)
+  if prefix == '+' or prefix == '-' or prefix == ' ' then
+    for i = lnum - 1, 1, -1 do
+      local prev_line = lines[i]
+      filename = parse_file_line(prev_line)
+      if filename then
+        local section = M.get_section_at_line(bufnr, i)
+        return filename, section
+      end
+      if prev_line:match('^%w+ %(') or prev_line == '' then
+        break
+      end
+    end
+  end
+
+  return nil, nil
+end
+
+---@param bufnr integer
+---@return string?
+local function get_repo_root_from_fugitive(bufnr)
+  local bufname = vim.api.nvim_buf_get_name(bufnr)
+  local fugitive_path = bufname:match('^fugitive://(.+)///')
+  if fugitive_path then
+    return fugitive_path
+  end
+
+  local cwd = vim.fn.getcwd()
+  local root = git.get_repo_root(cwd .. '/.')
+  return root
+end
+
+---@param vertical boolean
+function M.diff_file_under_cursor(vertical)
+  local bufnr = vim.api.nvim_get_current_buf()
+  local lnum = vim.api.nvim_win_get_cursor(0)[1]
+
+  local filename, section = M.get_file_at_line(bufnr, lnum)
+
+  if not filename then
+    vim.notify('[diffs.nvim]: no file under cursor', vim.log.levels.WARN)
+    return
+  end
+
+  local repo_root = get_repo_root_from_fugitive(bufnr)
+  if not repo_root then
+    vim.notify('[diffs.nvim]: could not determine repository root', vim.log.levels.ERROR)
+    return
+  end
+
+  local filepath = repo_root .. '/' .. filename
+
+  dbg('diff_file_under_cursor: %s (section: %s)', filename, section or 'unknown')
+
+  if section == 'untracked' then
+    vim.notify('[diffs.nvim]: cannot diff untracked file (no base version)', vim.log.levels.WARN)
+    return
+  end
+
+  commands.gdiff_file(filepath, {
+    vertical = vertical,
+    staged = section == 'staged',
+  })
+end
+
+---@param bufnr integer
+---@param config { horizontal: string|false, vertical: string|false }
+function M.setup_keymaps(bufnr, config)
+  if config.horizontal and config.horizontal ~= '' then
+    vim.keymap.set('n', config.horizontal, function()
+      M.diff_file_under_cursor(false)
+    end, { buffer = bufnr, desc = 'Unified diff (horizontal)' })
+    dbg('set keymap %s for buffer %d', config.horizontal, bufnr)
+  end
+
+  if config.vertical and config.vertical ~= '' then
+    vim.keymap.set('n', config.vertical, function()
+      M.diff_file_under_cursor(true)
+    end, { buffer = bufnr, desc = 'Unified diff (vertical)' })
+    dbg('set keymap %s for buffer %d', config.vertical, bufnr)
+  end
+end
+
+return M