Revisit whether resizeToVisibleArea is still necessary #66

Context

Rideau currently supports RideauContentContainerView.ResizingOption:

• .noResize: pins bodyView to the full container
• .resizeToVisibleArea: pins bodyView to visibleAreaLayoutGuide

However, RideauContentContainerView already exposes visibleAreaLayoutGuide and accessibleAreaLayoutGuide.
If content can attach its internal views to those guides directly, Rideau may not need to resize the entire bodyView.

Questions

• Is resizeToVisibleArea still necessary as a core API?
• Should it be treated as a convenience / compatibility mode?
• Would a guide-based layout model be clearer?
• Should ResizingOption be renamed, deprecated, or documented more explicitly?

Notes

resizeToVisibleArea is still useful for existing content that does not know about Rideau layout guides, especially scroll views whose viewport should match the visible sheet area.

For now, keep behavior unchanged and use this issue to track the design decision.