`Border` should support `ViewportSettings.Transparent` & `ViewportSettings.TransparentMouse`

[tig]

Summary

Border (an Adornment) does not currently honor ViewportSettingsFlags.Transparent or ViewportSettingsFlags.TransparentMouse. When these flags are set on a Border, the expected behavior is:

• Transparent: Areas of the Border that are not occupied by drawn border lines or title text should be visually transparent, allowing underlying views to show through.
• TransparentMouse: Mouse events in transparent areas of the Border should pass through to underlying views, while clicks on drawn border lines/title text are still handled by the Border.

Expected Behavior

When Border.ViewportSettings includes ViewportSettingsFlags.Transparent:

• Only the actual border lines and title text are drawn; empty areas within the Border's bounds are transparent.
• Underlying views are visible through the transparent areas.

When Border.ViewportSettings includes ViewportSettingsFlags.TransparentMouse:

• Mouse events in transparent (non-drawn) areas pass through to views underneath.
• Mouse events on border lines and title text are still captured by the Border.

Motivation

This is a prerequisite for the Border-based TabView redesign (#4183). In the new design, each Tab is a View whose Border renders a small tab header rectangle. The Border area outside the header must be transparent (both visually and for mouse events) so that:

1. The content of the selected tab is visible through the non-header portion of unselected tabs' Borders.
2. Clicks outside a tab header pass through to the content area below.

Current Behavior

Setting ViewportSettingsFlags.Transparent and/or ViewportSettingsFlags.TransparentMouse on a Border has no effect — the Border still fills its entire area and captures all mouse events within its bounds.

Acceptance Criteria

• Border with ViewportSettingsFlags.Transparent renders only border lines and title text; empty areas are transparent
• Border with ViewportSettingsFlags.TransparentMouse passes mouse events through transparent areas
• Unit tests covering both flags on Border
• Existing Border tests continue to pass