Add minimal Snow MPM simulation examples

[utilForever]

Summary

Add a minimal standalone Snow MPM example application under Examples/SnowMPMSim/ after #164 is merged.

The application should demonstrate that SnowMPMSolver3 can run end-to-end collision and deformation scenarios and write particle positions for external visualization.

Scope

• Add one SnowMPMSim executable with two selectable 3D scenes:
  1. A snow block falling and settling on frictional ground.
  2. A snow mass impacting a simple rigid obstacle and plastically deforming.
• Select the scene with --example 1|2.
• Write particle positions as one .xyz file per frame.
• Support only the essential command-line options: example, resolution, frame count, FPS, and output directory.
• Register the executable in the root CMake build.
• Reuse existing example patterns and dependencies; do not add a renderer or a new dependency.

Acceptance criteria

• The example target builds in a CPU-only configuration with BUILD_EXAMPLES=ON.
• A short run of each scene completes successfully and produces finite particle positions.
• Together, the scenes exercise rigid-body collision, friction, plastic deformation, and closed domain boundaries.
• --help documents the supported options.

Out of scope

• Additional scenes or a material preset system
• Mesh reconstruction
• Integrated rendering or playback
• Performance benchmarking

Blocked by #164.