ram_pcd_message: pass libff::bit_vector by const reference to avoid copies

[MozirDmitriy]

• Replace pass-by-value with const references for bit_vector parameters in ram_pcd_message constructor (root_initial, root, cpu_state, cpu_state_initial).
• Eliminates unnecessary copies while preserving API behavior and call sites.
• Aligns with prevalent codebase style where bit vectors are passed by const reference.