US-1.2.1: Comb polymer generation

[Peariforme]

User Story

As a polymer chemist, I want to generate a comb polymer with regularly spaced branches, to model architectures like PEG-comb.

Acceptance Criteria

• BranchedBuilder::comb_polymer(branch_every) works
• Backbone and branches can be different monomers
• Correct SMILES branching
• Mn includes backbone + branches

Files

• crates/polysim-core/src/builder/branched.rs (currently todo!())