Add missing Parcelable to InsideTheBackStack.NavTarget and IndexedBackStack.State

[LissF]

Summary

The sample app crashes when Android saves instance state (e.g. pressing Home while on the "Inside the backstack" screen). InsideTheBackStack.NavTarget and IndexedBackStack.State were missing Parcelable implementation, causing Parcel.writeValue() to fail at runtime with IllegalArgumentException: unknown type for value.

• Added Parcelable to InsideTheBackStack.NavTarget sealed class and @Parcelize to Child
• Added Parcelable to IndexedBackStack.State sealed interface and @Parcelize to all implementations

Steps to reproduce

1. Launch sample.app
2. Tap "Inside the backstack"
3. Press the Home button (triggers onSaveInstanceState)
4. App crashes with: java.lang.IllegalArgumentException: Parcel: unknown type for value 0

Maestro test

The following Maestro script can be used to verify the fix — it navigates to the screen, presses Home, resumes the app, and asserts the screen is still visible (no crash):

appId: com.bumble.appyx.samples
---
- launchApp
- assertVisible:
    text: "Inside the backstack"
- tapOn: "Inside the backstack"
- assertVisible:
    text: "Pop"
- pressKey: Home
- waitForAnimationToEnd
- launchApp:
    appId: com.bumble.appyx.samples
    stopApp: false
- assertVisible:
    text: "Pop"

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