Refactor fluid assembly to unified functions

This commit refactors the fluid assembly functions (fluid_2d_c, fluid_2d_m, fluid_3d_c, fluid_3d_m) into unified functions (fluid_unified_c, fluid_unified_m). This change aims to reduce code duplication and improve maintainability by consolidating the logic for both 2D and 3D cases. The legacy wrappers are kept for backward compatibility.

Co-authored-by: abrown97 <[email protected]>

Author: cursoragent