[barlowa124]

Fixes #1520. meshio.Mesh(points, cells, cell_data=...) crashed with KeyError: 0 when a cell_data value was a nested dict ({cell_type: {name: data}}) instead of the documented {name: [per-block arrays]} shape.

The validation loop checks len(data) != len(cells) and then indexes data[k]. A dict passes the length check whenever its key count happens to equal the block count, so the first subscript raises KeyError with no hint about the contract. The issue reporter hit exactly this with {"triangle": {"value": arr}} on a single triangle.

This adds a Mapping check at the top of the loop. That input now raises

TypeError: cell_data['triangle'] is a dict; cell_data must map a field
name to a list of arrays, one array per cell block.

TypeError because the signature already declares dict[str, list[ArrayLike]], so a mapping value violates the declared contract. The neighboring shape mismatches remain ValueError, matching the file's convention for value-level problems.

Regression test in tests/test_mesh.py covers the reported shape. List and ndarray values are unaffected. pytest tests/test_mesh.py reports 6 passed, 1 skipped.