Merge conflict UX: make u undo result-buffer edits from any conflict pane

[Copilot]

In merge mode, accept/discard actions update the result buffer, but undo previously required focus-switching into the result pane. This change makes u in non-result conflict panes undo the result buffer directly, enabling conflict-by-conflict workflows without navigation churn.

• Conflict keymap behavior

  • Adds a conflict-scoped, buffer-local u mapping on left/right merge panes.
  • Executes undo in the result buffer context, preserving result-pane undo semantics while removing pane-switch overhead.

• Regression coverage

  • Extends merge keymap ownership specs with a focused case that:
    • applies a conflict action from a non-result pane,
    • triggers u from that pane,
    • asserts result content returns to the pre-action state.

lifecycle.set_buf_keymap(tabpage, bufnr, "n", "u", function()
  if not session.result_bufnr or not vim.api.nvim_buf_is_valid(session.result_bufnr) then
    return
  end
  vim.api.nvim_buf_call(session.result_bufnr, function()
    vim.cmd("undo")
  end)
end, vim.tbl_extend("force", base_opts, { desc = "Undo previous result edit" }))

• Fixes Feat: pressing "u" undoes modification to result buffer regardless of active buffer location #271