Implement mesh patch material arrays and device transfer

[Copilot]

Mesh materials are transitioning from per-facet to per-patch granularity. This adds the infrastructure to store and transfer patch material data to device memory, parallel to existing facet arrays.

Changes

Type Definitions

• Added patchID_t (uint16_t) for patch identification

API-Level Arrays

• m_mesh_facet_patch - maps each facet to its parent patch
• m_mesh_patch_owner - owner ID per patch (flattened across meshes)
• m_mesh_patch_materials - material offset per patch
• nMeshPatches - total patch count
• Marked m_mesh_facet_materials as deprecated

Device Arrays (dT)

• triPatchID - patch ID per triangle
• ownerMeshPatch - owner per patch
• patchMaterialOffset - material offset per patch

Initialization

• Meshes without explicit patches default to single patch (ID 0)
• Patch materials derived from constituent facet materials
• Data flows through initGPUArrays → populateEntityArrays → device arrays

Example Usage

auto mesh = DEMSim.LoadMeshType("mesh.obj", material);
mesh->SplitIntoConvexPatches(30.0);  // Creates patches

// At initialization:
// m_mesh_patch_owner: [0, 0, 0, ...]  // One entry per patch
// m_mesh_facet_patch: [0, 0, 1, 1, 2, ...]  // Maps facets to patches
// m_mesh_patch_materials: [mat_id, mat_id, ...]  // One per patch

Force model integration (using triPatchID to index patchMaterialOffset) deferred to follow-up work.

Original prompt

Implicitly, all mesh patches have IDs, just like triangle facets. Now we need to do something about them. 1. We already have flattened arrays m_mesh_patch_owner and m_mesh_patch_materials (but are currently not used). These should now store flattened triangle mesh information at initialization, and then transfer the information to dT, very similar to the triangle facet info. Also, as now only patches have material properties, m_mesh_facet_materials should retire now and its usage is replaced by m_mesh_patch_materials. 2. We need to add m_mesh_facet_patch to store the facets' patch belonging info (which is useful in the force model when the model acquires material info; but the actual implementation of this can be done later). For now, just prepare the flattened info in these arrays and transfer to dT, similar to what we already do for facets.

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