Complete N4 bias field rescaling pipeline

[Copilot]

The N4 bias field correction pipeline had several incomplete TODOs: the calc_n4_rescaling rule in segmentation.smk had syntax errors and pointed to a non-existent script, and n4_biasfield.py had placeholder #TODO comments for reading scale/offset values.

Changes

• pyproject.toml: Switch zarrnii to branch copilot/add-scale-and-offset-options, which adds scale and offset parameters to N4BiasFieldApply

• segmentation.smk: Fix calc_n4_rescaling rule — extra leading space before input:, missing ) closing the mask=bids(...) call, added resources block, removed #TODO comment

• scripts/calc_n4_rescaling.py (new): Computes the linear scale/offset to map the bias-corrected intensity range back to the original input min/max within the brain mask, and writes them to a scaleoffset.txt file:

  # corrected * scale + offset => [input_min, input_max]
  scale = (input_max - input_min) / (corrected_max - corrected_min)
  offset = input_min - scale * corrected_min
  

• scripts/n4_biasfield.py: Reads scale/offset from the txt file (with key-presence validation) and passes them to N4BiasFieldApply:

  N4BiasFieldApply(log_space=True, scale=scale, offset=offset)