Render SwiftUI view modifiers on Android

[colemancda]

Summary

View modifiers previously had no effect on Android. ModifiedContent's AndroidPrimitive conformance was incomplete — it unconditionally flattened to the unmodified content, silently discarding every modifier. Only .opacity() worked, via a separate self-contained mechanism that never went through this path.

This wires up a real dispatcher and adds per-modifier conformances for the core layout and effect modifiers.

What's included

• ModifiedContent dispatcher — applies AndroidViewModifier conformances imperatively to the resolved native view, and reproduces the view-tree expansion for the background/overlay modifiers that genuinely build a distinct tree.
• AndroidCompositingContainer — a self-mounting FrameLayout host backing .background()/.overlay(). It creates and attaches children imperatively rather than via ParentView/reconciler diffing, because it's frequently reached through an outer modifier's own resolution path, which bypasses the reconciler entirely. Built to be reused for ZStack.
• Color as a native view — Color had no direct renderer (its Body was a shape-view composite with no native backing), so it couldn't be used as bare content for compositing.
• Modifier conformances — .padding(), .frame(), .offset(), .rotationEffect(), .scaleEffect(), .clipShape()/.cornerRadius().
• Gallery screen — ModifierScreen exercising each modifier individually.

Notes

• .background(Color.x) resolves to _BackgroundStyleModifier, not _BackgroundModifier — Swift picks the more specific ShapeStyle-constrained overload since Color conforms to both. Only solid Color styles render; gradients and materials need a Shape-backed renderer that doesn't exist yet, so they're skipped rather than rendered wrong.
• Clip handles rounded-rect and plain-rect only, the dominant real-world usage.
• .foregroundColor is not included — it has no dedicated modifier struct upstream and needs an environment-read hook in AndroidText instead.

Verification

All seven sections of the new gallery screen confirmed rendering correctly on the emulator: padding, frame, background, clip + corner radius, offset, rotation, and scale.