Checkout the artwork from both cards - Stoneforge Mystics and Nahiri, the Lithomancer, and you will see no difference between the two people. The clothing is exactly the same and she is holding a sword in the planeswalker version. What's the tie-in between the two cards? Let's find out.
Nahiri sealed the Eldrazi's into a Perilious Vault with Sorin and Ugin 6000 thousand years ago on Zandikar. Stoneforge Mystic was printed in worldwake which is a set printed after Zandikar. The premise of the story is that the characters are discovering why the world is awakening. If Nahiri is Stoneforge Mystic, wouldn't she just wake up one day and say "Hey guys, I think the world is awakening because we FREAKING SEAL THE ELDRAZI IN OUR BACKYARD!"?
"She stared up at the silent, stony shapes. Ramparts of stone already crept up around them. Perhaps in a thousand years, they would be forgotten, their destruction fading into legend. But Nahiri would not forget them, and neither would the land itself."
Based on that, I highly doubt that the Nahiri is THE Stoneforge Mystic but she might have started an order of Lithomancer and Stoneforge Mystic is one of them.
If Nahiri was not in the Stoneforge Mystic, then she must have gone somewhere after she sealed off the Eldrazi, where could she be? There is so much mystery to be solved here.