How much will a bad 1st semester junior year hurt me for MIT/Stanford/Princeton/Caltech/Cal?

https://www.universityofcalifornia.edu/infocenter/transfers-major indicates that the 25th-75th percentile college GPAs and admit rates for transfers are:

3.88-4.00, 08%, UCB EECS
3.81-4.00, 07%, UCB L&S CS

Note that UCB does not offer TAG.

Most traditional students do not earn as high a college GPA as their unweighted high school GPA.