Over the years, phone-makers have shown off handsets that stretch, bend and fold. But inside a secret room at Samsung Display’s headquarters in South Korea — one that had never before been opened up to the press — I got a firsthand look at the company’s vision for the future of smartphones.Along one wall sat a lineup of concept displays that could eventually make their way into future mobile devices. I wasn’t allowed to touch the prototypes, but they were mounted on machines that repeatedly bent, folded and rolled the displays, showing off their range of motion. This story is…