scroll: Fix markdown table horizontal scroll leaking into vertical body scroll (#2608)

## Description

With a trackpad, horizontal swipes over a horizontally scrollable
markdown table (`TextViewStyle.table` with `overflow_x: scroll`)
scrolled the whole markdown body vertically instead, making the table
nearly impossible to scroll. Two causes:

1. With `scrollable(true)` the body is a `gpui::list`, which registers
its wheel listener after its items paint, so in the bubble phase
(reverse registration order) it consumed `delta.y` before
`ScrollableMask` could stop the propagation. The mask now consumes
axis-dominant wheel events in the capture phase; vertical-dominant
events keep bubbling to the body (preserves #2468).

2. The mask was a child of the scrolled element and slid away from the
viewport as the table scrolled, leaving the uncovered part leaking to
the parent scroller. It is now a sibling of the scrolled element, same
as `Table`.

The mask also checks `HitboxId::should_handle_scroll` now, staying inert
when occluded (dialogs, context menus). Since capture runs in
registration order, nested masks resolve innermost-first.

Added 4 tests reproducing the `gpui::list` structure.

Co-authored-by: Claude Fable 5 <noreply@anthropic.com>