Write channel 2 registered tiffs to reg_tif_chan2

[adityasingh2400]

When a two-channel recording is registered with both reg_tif and reg_tif_chan2 enabled, the second channel's tiffs are written into the first channel's directory and overwrite it. The reg_tif_chan2 folder is created but stays empty, and the tiffs left in reg_tif contain channel 2 data.

assign_reg_io already computes two separate tiff directories, tif_root_align for the alignment channel and tif_root_alt for the alternate one, and picks the right folder for each depending on align_by_chan2. registration_wrapper passes tif_root_align to register_frames, which is correct, but then passes tif_root_align again to the shift_frames_and_write call that handles the alternate channel. Both channels therefore write file 0000.tif into the same folder, and since the alternate channel is written second it wins. tif_root_alt was computed and never used.

The fix passes tif_root_alt to the alternate channel write. This also repairs the case where only reg_tif_chan2 is set, where tif_root_align is None and the alternate channel silently wrote no tiffs at all. The bug is symmetric under align_by_chan2, since the roles of the two folders swap but the wrong variable is used either way.

Tested with a new parametrised case in tests/test_registration.py. It runs registration_wrapper on an 8 frame 64 by 64 synthetic pair whose two channels differ by an order of magnitude in mean intensity, so the mean of a written tiff identifies which channel produced it, and asserts that each directory holds its own channel. Both parameter values fail on main, one because reg_tif_chan2 is empty and the other because reg_tif holds the wrong channel, and both pass with this change. The rest of the file still passes, 5 passed in total.

Fixes #1208