[iOS] iOS handlers for all page types (FlyoutPage, TabbedPage, NavigationPage)

[PureWeen]

iOS Handler Unification Epic

Target: .NET 10

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Description

Unify iOS navigation containers (NavigationPage, TabbedPage, FlyoutPage) to use shared handler implementations that Shell also uses. This aligns iOS with WinUI, which already has unified handlers, and sets the pattern for Android (.NET 11).

Currently, iOS has legacy renderers (NavigationRenderer, TabbedRenderer, PhoneFlyoutPageRenderer) that duplicate logic found in Shell's iOS implementation. Bug fixes and improvements must be applied in multiple places, leading to inconsistent behavior and double maintenance burden.

By creating unified handlers that both the standalone page types and Shell consume, we reduce code duplication, improve consistency, and make the codebase easier to maintain and contribute to.

All handlers will be gated behind AppContext switches for safe rollout.