fix(scrollview): honor padded viewport

[cdd1037]

Summary

• define one padding-aware inner viewport for ScrollView layout, scrolling, thumb geometry, and hit testing
• clip scrolled content to that inner viewport so it cannot paint into the padding area
• draw the overlay scrollbar after content so opaque children cannot cover it
• add a regression test for padded viewport overflow and maximum scroll range

Bug

DoLayout() subtracted padding, while VisibleHeight(), clamping, and thumb calculations still used the outer widget height. A 100 px ScrollView with 10 px top/bottom padding therefore treated its viewport as 100 px instead of 80 px. Content near that boundary could fail to show a scrollbar or could not be scrolled fully into view.

The scrollbar was also painted before the child tree, despite being an overlay, so opaque content could cover it.

Validation

The regression test uses a 100 px outer height, 10 px vertical padding on both sides, and 90 px content. It verifies that a scrollbar is required and the maximum scroll offset is 10 px. The test and optimized Windows x64 build pass under clang-cl.

Build: https://github.com/cdd1037/core-ui/actions/runs/32629855670