chore(guide): bump vtk-wasm bundle to 9.6.20260621

[jspanchu]

Only one example shows errors:

const vtk = await vtkwasm.ready;
const coordinates = vtk.vtkTypeFloat64Array();
coordinates.number_of_components = 3;
await coordinates.SetArray(new Float64Array([-1, -1, 0, 1, -1, 0]));
console.log(await coordinates.GetTuple1(0));

Errors:

2026-06-22 09:43:19.942 (   0.001s) [          939654]      vtkSerializer.cxx:86     ERR| vtkSerializer (0x95f4c8): Failed to serialize objectBase=vtkTypeFloat64Array (0x98c258). message=[json.exception.out_of_range.403] key 'MTime' not found
2026-06-22 09:43:19.942 (   0.002s) [          939654]   vtkObjectManager.cxx:885    ERR| vtkObjectManager (0x95f3e0): Failed to update state for object at id=1
2026-06-22 09:43:19.942 (   0.002s) [          939654]      vtkSerializer.cxx:86     ERR| vtkSerializer (0x95f4c8): Failed to serialize objectBase=vtkTypeFloat64Array (0x98c258). message=[json.exception.out_of_range.403] key 'MTime' not found
2026-06-22 09:43:19.942 (   0.002s) [          939654]   vtkObjectManager.cxx:885    ERR| vtkObjectManager (0x95f3e0): Failed to update state for object at id=1
-1

Will create issue separately to track bugfix for that.