I'm completely odd, except at my start. I go on forever, although humanity has mapped me to forty-one million numbers in length. While no pattern exists to define me, a theorem exists to predict my distribution. This riddle reads hard, but truly I'm the only divisible answer.

I can be made of stone, language, or sound, Built to keep limits or keep you aground. Break me to pass or stay on your side, I’m the thin line where two worlds collide.

I'm cold when I'm full, Hot when I starve. When you want your whisky neat, You'll need what my tummy keeps.

Remove my last two, and I become someone just fitting in - one with my surroundings. Remove my first, and never a borrower nor this be. Altogether, I mix and stir, all in a blur - until everything is smooth and pureed.

I am not female nor male, but I share in name. I am not plate or pad, but I am in-between. I am not whole, but I am forged ring by ring. I am not weak, I am but steel. Wear me now, for I save from others steel. What am I?