[Feature] Sand (granular) simulation: loose grains that roll and pile

Motivation

The solver currently handles shells, solids, and rods, but not granular media. Sand, gravel, and similar piling/pouring effects are common in animation and product visualization, and there is no way to produce them today. Adding a granular body type would let users simulate grains falling, rolling, and piling under gravity with frictional contact, while still interacting with the existing shell/solid/rod objects in the same scene.

Proposal

Add a sand body type: a collection of loose spherical grains (a faceless point cloud) that the solver advances under gravity with grain-grain, grain-wall, and grain-object frictional contact. Grains should roll, not just slide, so piles form at realistic angles of repose.

Suggested user-facing surface:

• A SAND group / object type.
• A way to turn a closed mesh into a particle cloud (volume seeding), or to feed a point cloud directly.
• Tunable parameters: grain radius, particle mass, and friction.
• Grains collide with existing shells, solids, and rods in the same scene.

Alternatives Considered

• Approximating grains with many small solid/shell bodies. Impractical at grain counts and does not capture rolling/piling cleanly.
• Doing granular effects in a separate package and compositing. Loses the two-way contact with the cloth/solid objects already in the scene.

Additional Context

Requested as a granular capability to complement the existing shell/solid/rod support, with two-way coupling to those object types.

Contribution Agreement

• I confirm that any code, patches, scripts, attachments, or other copyrightable material I include in this issue may be used, reproduced, modified, and distributed by this project under the Apache License 2.0 or as otherwise described in CONTRIBUTING.md.
• This is a feature request from the community (please provide a reference below).

Community Reference

No response

No source files, tests, or entry points are identified. Begin by locating the existing shell, solid, and rod object types and solver contact handling; done would require a sand object type with volume or point-cloud input, tunable grain parameters, rolling and piling behavior, and interaction with those existing objects.