Bugfix: Preserve ForEach TabView State

[fhasse95]

This PR fixes a TabView state reset on Android when tab content is produced by an unrolled ForEach.

This appears to be a regression after #500. ForEachIdentityModifier keyed the rendered subtree with both the remembered ForEach namespace and the element identity. In TabView, content can leave and re-enter the composition, recreating that namespace and causing stable tab content to remount.

This change keeps the namespace for parent container child-key calculation, but uses only the stable element identity for the modifier's inner render key.

To reproduce:

import SwiftUI

struct RootTabView: View {
    var body: some View {
        TabView {
            ForEach(0..<3, id: \.self) { tab in
                TabContentView(tab: tab)
                    .tag(tab)
                    .tabItem {
                        Text("Tab \(tab)")
                    }
            }
        }
    }
}

private struct TabContentView: View {
    let tab: Int
    @State private var instanceID = String(UUID().uuidString.prefix(4))
    @State private var tapCount = 0

    var body: some View {
        VStack(spacing: 16) {
            Text("Tab \(self.tab)")
                .font(.title)
            Text("Instance: \(self.instanceID)")
                .font(.headline)
        }
    }
}

Before:

After:

=> As you can see, before this change the instanceID state changes every time the tab is revisited, because the tab content is remounted. After this change, the instanceID remains stable when switching away from a tab and back.

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[fhasse95]

Hi @marcprux, thanks for merging my other PRs.

However, after merging the latest main into my temporary branch, I noticed this small Android regression. It seems related to #500 and affects TabView content produced by an unrolled ForEach, where local tab states can reset when switching tabs.

The fix is tiny and the PR includes a small repro plus before/after screenshots. If possible, it would be great to get this into the next skip-ui release as well. Thanks! 😊