Fixes #4843. Button: avoid create-then-destroy shadow allocation via CWP InitializingShadowStyle event

[Copilot]

Button unconditionally allocated shadow infrastructure (MarginView + 2 ShadowViews, ~23KB) in its constructor, then subclasses like ScrollButton and ArrangerButton immediately nulled the shadow — destroying the ShadowViews but leaking the MarginView (~7KB) and generating ~69KB of transient GC pressure per instance.

Proposed Changes/Todos

• Button.cs — adds protected virtual void OnInitializingShadowStyle(ValueChangingEventArgs<ShadowStyles?> args) (empty in base) and public event EventHandler<ValueChangingEventArgs<ShadowStyles?>>? InitializingShadowStyle, following the Cancellable Work Pattern (CWP). The private RaiseInitializingShadowStyle() method orchestrates the CWP flow: creates args with NewValue = DefaultShadow, calls the virtual, raises the event, then applies args.NewValue (unless args.Handled). This allows subclasses and external subscribers to change or suppress shadow allocation before any infrastructure is created.

• ScrollButton.cs — overrides OnInitializingShadowStyle to set args.NewValue = null; removes the now-redundant base.ShadowStyle = null from its constructor. The base constructor then calls base.ShadowStyle = null, which is a no-op — no MarginView or ShadowViews are ever created.

• ArrangerButton.cs — same pattern as ScrollButton.

// Before: Button() creates shadow, ScrollButton() destroys it → 69KB GC pressure, 7KB leak
public ScrollButton() {
    base.ShadowStyle = null; // too late — shadow already created in Button()
}

// After: no shadow infrastructure created for ScrollButton
protected override void OnInitializingShadowStyle(ValueChangingEventArgs<ShadowStyles?> args)
    => args.NewValue = null;

Memory impact

Type	Before	After	Saved
ScrollButton	~67KB	~9KB	~58KB
ArrangerButton	~78KB	~9KB	~69KB
ScrollBar (2 buttons)	~230KB	~116KB	~116KB

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