Use ImageRenderer for ideal size layout on iOS 16+

[abrahamduran]

Fixes #1070

Summary

.sizeThatFits is broken on modern iOS simulators. UIHostingController.sizeThatFits(in: .zero) returns the view's minimum compressed size instead of its ideal size, causing truncated text and collapsed layouts.

The root cause: CGSize cannot express "unspecified" dimensions. SwiftUI's ProposedViewSize supports nil (meaning "use your ideal size"), but there's no CGSize equivalent — so there's no value you can pass to UIHostingController.sizeThatFits(in:) that means "ideal size."

Changes

SwiftUIView.swift — For .sizeThatFits on iOS 16+, use ImageRenderer with ProposedViewSize(width: nil, height: nil) instead of the UIHostingController → UIWindow → layer.render pipeline. This is the SwiftUI-native way to ask for ideal size.

• ImageRenderer bypasses UIKit's trait system, so UITraitCollection.userInterfaceStyle is converted to SwiftUI's .colorScheme environment
• MainActor.assumeIsolated is used because ImageRenderer is @MainActor-isolated but the asyncPullback closure is nonisolated (snapshot tests always run on main thread)
• .device and .fixed layouts are unchanged — they still use the UIHostingController/UIWindow path

iOS 13-15 fallback — Uses screen width with unconstrained height as a reasonable heuristic, since ImageRenderer isn't available.

Tests — Two new tests that expose the bug:

• testSwiftUIView_sizeThatFits_padding — Text with .padding() (padding collapses with the old path)
• testSwiftUIView_sizeThatFits_spacer — HStack with Spacer() (spacer gets crushed to zero with the old path)

Note on baselines

The testSwiftUIView_iOS.size-that-fits.png baseline was re-recorded with the new ImageRenderer path. The new test baselines were recorded on an iPhone 17 simulator (@3x) — they may need re-recording on your CI's pinned simulator.