Add MolBuilder - pure-Python molecular engineering and process chemistry toolkit

[Taylor-C-Powell]

Adding MolBuilder, an Apache 2.0-licensed Python package for molecular engineering and process chemistry. Key features:

• Pure Python (numpy/scipy/matplotlib only) -- no RDKit or C++ required
• SMILES parser/writer, 21 functional group detectors
• Retrosynthetic planner with 91 reaction templates
• Process engineering: reactor selection, condition optimization, GHS safety, costing, scale-up
• 1,280+ tests, CI on Python 3.11/3.12/3.13

PyPI: https://pypi.org/project/molbuilder/