Add recipe showing how to model more complex back stack behavior

[dturner]

WIP

[gemini-code-assist]

Summary of Changes

Hello @dturner, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

I've added a new recipe to demonstrate more complex back stack behavior in Android Compose using a custom Navigator class. This recipe provides a flexible way to manage navigation, including nested and shared routes, and showcases how to integrate it with a NavDisplay.

Highlights

• Custom Navigator Class: Introduced a custom Navigator class in Navigator.kt to model advanced navigation behavior, including a mutable back stack, top-level routes, and shared routes.
• Example NavigatorActivity: Implemented NavigatorActivity.kt as a practical example, demonstrating how to use the Navigator with a bottom navigation bar and a search functionality.
• Unit Tests for Navigator: Added comprehensive unit tests in NavigatorTest.kt to validate the Navigator's behavior across various scenarios, such as navigating to top-level routes, handling shared routes, and managing the back stack.
• Build Configuration Update: Updated app/build.gradle.kts to include the Kotlin test dependency for the new unit tests.
• Android Manifest Update: Registered the new NavigatorActivity in app/src/main/AndroidManifest.xml.

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[gemini-code-assist]

Code Review

This pull request introduces a new recipe for modeling complex back stack behavior using a custom Navigator class. The implementation is well-structured with a corresponding activity and unit tests. My review focuses on a critical bug in the back navigation logic that could lead to a crash, along with several medium-severity issues related to code robustness, performance, accessibility, and test clarity. Addressing these points will improve the quality and reliability of the new recipe.