US-1.1.3: Block copolymer generation

[Peariforme]

User Story

As a polymer chemist, I want to generate a block copolymer (AAAA-BBBB) with specified block lengths, to model systems like PS-b-PEO.

Acceptance Criteria

• LinearBuilder::block_copolymer(&[usize]) accepts N blocks with lengths
• Support 2+ blocks (diblock, triblock, multiblock)
• Correct ring closure handling between blocks
• Tests: PS-b-PMMA diblock, PS-b-PB-b-PS triblock (SBS)

Files

• crates/polysim-core/src/builder/linear.rs (line ~106, currently todo!())