Compute net charge versus pH over a chosen window, plus the isoelectric point. Use for buffer and formulation-pH selection, for predicting IEX binding and elution behaviour, and for reading a pI shift between a reference and a comparability lot.

Parameters:
  sequence: Protein amino acid sequence.
  ph_start: Lower bound of the pH window (default 2.0).
  ph_end: Upper bound of the pH window (default 12.0).

Returns: {charge_profile: [{ph, charge}], isoelectric_point}