Add scroll support to inspector component groups #18

The selected-entity inspector needs scroll support for component groups whose content exceeds the available inspector panel height.

Planned scope:

• Keep inspector layout/input routed through reusable retained UI or shared editor layout paths.
• Make overflowing component group content scrollable and clipped.
• Add deterministic editor input coverage for wheel scrolling.
• Verify the editor render path remains healthy.

This tracks the docs/TODO.md Next Slice item: "Add scroll support to inspector component vgroups."