Refactor case BC hooks: rename to define_BC/apply_BC and hoist BC fields to base

[kaanolgu]

• Rename boundary_conditions -> define_BC and pre_correction -> apply_BC across base and child cases.
• Hoist 12 BC field pointers (bc_{start,end}_{u,v,w}_{x,y}) onto base_case_t; cases use only what they need.
• Add field_set_face_from_field backend hook (CUDA + OMP) for spatially-varying face BCs.
• Channel: y-wall BCs now use persistent device fields refreshed with noise per substep, applied via the new hook.
• Cylinder: inflow profile now built field-side using new inlet_noise(3) config (replaces bc_start_u/v/w scalars).
• Cylinder: compute_outflow_params moved from apply_BC into define_BC; results stored on self%out_vel / self%flow_rate_diff (drops _cached suffix).
• Behavioural note: outflow params sampled one substep earlier than before.
• Breaking change: existing cylinder_nml files must replace bc_start_u/v/w with inlet_noise.