Add ScrollView to Pixely.Ui with wheel routing and overlay scrollbars

[botoddly]

Adds ScrollView, a container that shows a window onto children larger than itself, scrolled by the mouse wheel, by its bars and from code. The design was reviewed by codex before implementation.

ScrollView is a Column that clips, with the children slid by ScrollOffset: its Layout is the ordinary one and may be replaced, so a horizontal strip is a scroll view with a horizontal stack and ScrollAxes.Horizontal.

• Axes (measure property) says which axes scroll; on those the children are measured without a bound, so Grow and Percent degrade there as documented and Grow on the cross axis fills a definite width as in a column
• ScrollOffset is an arrange property, clamped into [0, MaxScrollOffset] at the next build, so int.MaxValue means the end once laid out; ScrollBy moves within the last build's range synchronously and reports whether it moved
• padding scrolls with the children, so the extent is children plus padding and the clip stays the element's bounds
• overlay bars along the trailing edges, ScrollBars Auto/Hidden/Visible, never on an axis that does not scroll; a track press pages by one viewport; thumb dragging is left for a later move notification
• ScrollAppearance on UiStyle.Scroll: Thumb per state, optional Track, Thickness, MinimumThumbLength

Wheel input:

• IScrollTarget.OnScroll(position, delta) returns the ScrollAxes it took; refusing means an outward delta at an end or an unsupported axis, and taking includes banking a touchpad fraction too small to move a pixel
• UiInputSystem subscribes to the wheel and UiRoot.PointerScrolled routes it: the topmost element of either kind under the pointer, then up through its ancestors, each taking the axes it can use, so a dialog over a list does not scroll it, a wheel over a button inside a list does, and a horizontal strip inside a vertical list splits a mixed delta between the two; consumed only when something took some of it
• the hit list carries a HitKind per entry, so a scroll-only element stays transparent to the pointer

Element changes:

• ResolveContentConstraints virtual, called once per measure, is what ILayoutHost resolves every child's sizing against; the default commits what was offered
• internal adornments (AttachAdornment, Adornments): owned elements outside Children, painted and hit-collected after them, never laid out, invalidated with the subtree; the bars use it, so Children.Clear() cannot reach them

Also docs/ui.md (Scrolling section, wheel and style notes), the Pixely.Tutorials.UiScrollView tutorial, and 45 tests across ScrollViewTests, ScrollRoutingTests and UiInputSystemTests. The tutorial was verified headless with screenshots: wheel over the list, paging by a track press, hover following the wheel, a click on a button scrolled into view.

[github-actions]

📦 Development package Pixely 0.0.60 is available.

Workflow run (attempt 1)

[github-actions]

✅ Development package validation passed on all platforms.

Workflow run (attempt 1)