Implement explicit 2-D and 3-D weakly compressible MPM fluid solvers

[utilForever]

Blocked by #166 and #167.

Goal

Assemble the EOS model and fluid transfer state into explicit CPU MPMFluidSolver2 and MPMFluidSolver3 implementations.

Scope

• Initialize particle reference volumes and density state.
• Accumulate pressure, gravity, and other external forces on active grid nodes.
• Advance nodal velocities with explicit integration.
• Transfer velocities back and update particle positions and volumes.
• Select stable substeps from grid spacing, velocity, and the configured speed of sound.
• Provide emitters and builders consistent with existing CubbyFlow solver APIs.

Acceptance criteria

• Empty updates are safe in both dimensions.
• With zero pressure and external forces, uniform particle motion is preserved within numerical tolerance.
• Gravity produces the expected total momentum change.
• Compressed particles accelerate away from the compressed region.
• Focused 2-D and 3-D unit tests pass without non-finite state.
• CPU-only builds remain independent of CUDA.

References

• https://doi.org/10.1016/0010-4655(94)00170-7