Fix display of multi-valued cluster metadata

[rossant]

Summary

• Display list-, tuple-, and NumPy-array-valued metadata as comma-separated text in native cluster tables.
• Preserve native NumPy scalar conversion.
• Add a focused ClusterView regression test and changelog entry.

Root cause

The Qt table model returned collection values directly for Qt DisplayRole. Qt renders those unsupported values as empty cells, unlike the previous web table.

Validation

• 46 native table and focused ClusterView tests passed.
• make lint
• make format-check
• Documentation link check and strict MkDocs build

Fixes #1402.