Implement incompressible 2-D and 3-D MPM fluid solvers

[utilForever]

Blocked by #169.

Goal

Extend the validated weakly compressible MPM fluid solver with an incompressible pressure projection in both 2-D and 3-D.

Scope

• Retain the weakly compressible solver as the reference and fallback path.
• Build pressure degrees of freedom over the active fluid region.
• Project grid velocity to satisfy the discrete incompressibility constraint.
• Apply free-surface pressure and solid-wall normal-velocity boundary conditions.
• Reuse the particle-grid transfers, collision handling, and active-grid representation from Integrate MPM particle-grid transfers for fluids #167–Add collisions and validation for weakly compressible MPM fluids #169.
• Document the selected pressure discretization and linear-solver convergence behavior.

Acceptance criteria

• Pressure projection reduces discrete grid divergence to the configured solver tolerance.
• A closed-box case preserves fluid volume within numerical tolerance.
• Hydrostatic and dam-break cases show lower density drift than the weakly compressible baseline.
• Free surfaces and solid boundaries use the intended pressure boundary conditions.
• Solver failure does not commit non-finite particle or grid state.
• Focused 2-D and 3-D unit tests pass.

References

• https://doi.org/10.1016/j.jcp.2016.10.025