Preserve PySide TreeView selection/expansion across keyed reorders

[berendkleinhaneveld]

What this changes

• Preserve QTreeView item UI state (selection and expansion) when keyed v-for rows are reordered in QStandardItemModel.
• Add snapshot and restore handling in:
  • collagraph/renderers/pyside/objects/itemmodel.py
  • collagraph/renderers/pyside/objects/standarditem.py
• Ensure deferred selected and expanded attributes are applied correctly when items are inserted.
• Add focused regression tests in tests/pyside/test_treeview_keyed_selection.py for:
  • bound selected and expanded surviving reorder
  • manual parent selection surviving reorder
  • manual child selection surviving parent reorder
• Add a trimmed manual demo: examples/pyside/keyed_tree_demo.cgx.

Why

Keyed reconciliation already preserves fragment identity, but Qt model/view state can still be lost when items are detached and reinserted during reorder operations. This PR preserves that UI state so moved items keep the same selection and expansion behavior.

Verification

• uv run pytest tests/pyside/test_treeview_keyed_selection.py -q
• Commit hooks passed on commit (lint, format, pytest).

Notes

This PR is a reimplementation of the intent from #142 against the current codebase architecture, rather than a direct application of the old patch.