refactor(test): add scenario for test-stack-simple-nav

[LKuchno]

Description

Closes: https://github.com/software-mansion/react-native-screens-labs/issues/1601

This PR adds a comprehensive manual test scenario for the gamma/v5 StackContainer's simple push/pop navigation, and adjusts the test-stack-simple-nav screen so the scenario can be exercised on both platforms.

The scenario documents the full lifecycle from root (Home) through stacking/unstacking screens A and B, covering:

• push/pop via the on-screen buttons,
• the native header back button,
• the iOS edge-swipe-back / Android system gesture-back.

It validates that each push creates a new instance with a unique routeKey, that a preserved (not recreated) screen keeps its routeKey on pop, and that the root screen (Home) cannot be popped. Edge cases (re-pushing an already-present route, rapid tapping push/pop, route-key uniqueness) are included.

The scenario also documents the platform launch difference: iOS runs from the in-app scenario menu, while Android must be launched directly via apps/App.tsx (rendering TestStackSimpleNav as the root). This is the current workaround for the Android back-button limitation (issue #1459), where the native back button and system gesture-back only pop the stack when the StackContainer is at the root rather than nested inside the example app's navigation.

Changes

• Test screen (index.tsx): Added headerConfig with a title to the Home, A, and B routes. Rendering a header makes the native back button appear on non-root screens (including on Android), which is required to test the header-back-button and gesture-back flows.
• Scenario metadata (scenario-description.ts): Renamed the scenario from "Simple navigation scenario" to "Simple stack navigation".
• Scenario (scenario.md): Added the manual scenario documenting all push/pop flows, native header-back-button and edge-swipe / system gesture-back interactions, routeKey behavior, per-platform launch instructions, and edge cases (re-pushing the same route, rapid tapping, root-screen protection). E2E automation is noted as TBD.

[Copilot]

Pull request overview

Adds a manual test scenario document for the Stack v5 “simple navigation” screen in the example app, describing expected push/pop behavior and platform-specific back navigation interactions.

Changes:

• Added a new scenario.md documenting push/pop flows across Home/A/B, including re-push behavior and routeKey expectations.
• Documented iOS-native back interactions (header back + edge-swipe) and Android limitations, plus E2E automation notes.

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[kkafar]

In general I'm against naming such details as background colours in the scenario description. It can very easily go out of date.

This is a minor however. I recommend to think about it.