Performance optimizations across core components

[Copilot]

Description of Change

Targeted performance fixes for five hotspots found during a codebase performance review.

• Touch.cs RemoveEffects — O(n²) while/FirstOrDefault/Remove loop → single-pass reverse for with RemoveAt(i)
• SearchBar.cs — CancellationTokenSource leaked on every keystroke; now disposed before replacement
• ListItem.cs OnVerticalStackLayoutSizeChanged — .Where().ToList().IndexOf() allocated a list on every SizeChanged event; replaced with zero-allocation foreach that breaks on first visible item
• DUI.cs EnsureSkLottieResourcesAdded — LINQ .All() scan on every call; cached with static bool after first registration
• Shell.cs debug diagnostics — two .Where().ToList() passes over the same collection; combined into single-pass partition loop

// Before: O(n²) — repeated linear scans
while (view.Effects.Any(e => e is Touch))
    view.Effects.Remove(view.Effects.FirstOrDefault(e => e is Touch));

// After: O(n) — single reverse pass
for (var i = view.Effects.Count - 1; i >= 0; i--)
    if (view.Effects[i] is Touch)
        view.Effects.RemoveAt(i);

Todos

• I have tested on an Android device.
• I have tested on an iOS device.
• I have supported accessibility

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