Add collisions and validation for weakly compressible MPM fluids

[utilForever]

Blocked by #168.

Goal

Complete the weakly compressible MPM fluid baseline with collision handling and compact physical validation scenarios.

Scope

• Apply closed-domain boundary conditions on active MPM grid nodes.
• Reuse CubbyFlow colliders for separating and frictional contact where supported.
• Resolve remaining particle penetration after grid-to-particle transfer.
• Add small hydrostatic and dam-break validation cases.
• Document the expected density variation and time-step limits of the weakly compressible baseline.

Acceptance criteria

• Focused unit tests cover domain walls and collider contact in 2-D and 3-D.
• A hydrostatic case remains bounded without non-finite density or pressure.
• A small dam-break case conserves particle mass and stays within the domain.
• Density error and total momentum are reported by the manual validation case.
• The change does not add rendering or output-format dependencies.

Follow-up criterion

Create an incompressible MPM issue only if the measured density error, pressure oscillation, or explicit time-step restriction makes the validated baseline unsuitable.