scrollable_mask: Keep rounded scroll-table corners clean at any scroll offset

[ihavecoke]

Problem

#3309 rounds the table's first and last rows, which only lines up while the content sits at its unscrolled position. Once a wide table scrolls horizontally, gpui's rectangular content mask slices the clipped side square into the frame's rounded corner notches — the known limit noted in #3309.

Fix

Add RoundedFrameCover, a sibling overlay (like ScrollableMask) pinned to the viewport: it fills each corner notch — inside the frame's rect corner, outside the border arc — with a backdrop color, so the frame reads as a rounded clip at every scroll offset. The frame's own border paints after its children, so it stays crisp on top of both the scrolled content and the covers; nothing is repainted.

horizontal_scroll_area installs the cover from the caller style's corner radii, so markdown tables get it automatically. The element is exported from gpui-base — RoundedFrameCover::uniform(radius), .backdrop(color), .frame_border(width) — so application frames with their own rounded scroll viewports can reuse it; frame_border grows the notches out to the border box when the cover is a child of the bordered element itself.

Before

After

Test

• corner_notch_paths_stay_inside_their_corner_squares pins the notch geometry: each path tessellates to an r-sized square anchored at its frame corner.
• cargo nextest run -p gpui-base scrollable_mask — 17 passed.
• cargo clippy -p gpui-base --locked -- --deny warnings / cargo fmt -- --check — clean.
• Verified end-to-end in a consuming application via a local patch: markdown tables and an application-owned scrollable card keep rounded corners through horizontal scrolling.