Document that multi-topology meshes are still unsupported and replace the old
`Physics` wording with kernel-centric language about shared meshes.
- Swap the future `MixedMesh` bullet for an explicit limitation note.
- Describe mesh ownership vs kernel references for multiphysics reuse.
Create src/mesh/api.jl defining mesh-specific abstractions:
- AbstractMesh base type for all mesh structures
- nnodes_total(), nelements() for mesh sizing
- get_node(node_id) returns Vec{Dim} coordinates
- connectivity_matrix() returns element-to-nodes mapping
- get_elements_for_node(node_id) returns node-to-elements mapping (critical for nodal assembly)
- get_element_set(name), get_node_set(name) for named sets (BCs, materials, postprocessing)
- AbstractRefineStrategy, refine() for adaptive mesh refinement
Meshes own topology (coordinates, connectivity). Multiple Physics can
share one Mesh for multiphysics coupling. Node-to-elements mapping
enables nodal assembly pattern (see docs/book/multigpu_nodal_assembly.md).
Part of systematic modular API architecture.
Jukka Aho