US-1.1.4: Gradient copolymer generation

[Peariforme]

User Story

As a polymer chemist, I want to generate a gradient copolymer where composition varies progressively along the chain.

Acceptance Criteria

• New method LinearBuilder::gradient_copolymer(profile: GradientProfile)
• Profiles: linear, sigmoid
• Monomer A fraction transitions from f_start to f_end along the chain
• Statistical verification of gradient monotonicity