Is ChaCha12 considered 256-bit secure?
Yes, the best attack still seems to be on 7 rounds. Namely, "Improved Key Recovery Attacks on Reduced-Round Salsa20 and ChaCha" shows a $2^{246.5}$ time attack on the 7-round variant.

So even the 12-round variant has a decent security margin – better than AES-256 had when standardized, much less currently.