feat(admob): scaffold next-gen sample app

[polmiro]

Summary

Scaffolds a focused example app for RevenueCat tracking with Google Mobile Ads Next-Gen:

• add the next-generation AdMob sample module and project wiring
• initialize RevenueCat and Google Mobile Ads for the sample
• add navigation for the format-focused examples

Screenshots

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Note

Low Risk
New isolated example module with no changes to core SDK behavior; format demos are stubs only.

Overview
Adds a new examples/admob-next-gen-sample Compose app and registers it in settings.gradle.kts, aimed at demonstrating RevenueCat ad-event tracking with the Google Mobile Ads Next-Gen SDK via :purchases and :feature:admob-next-gen.

The scaffold wires RevenueCat and Mobile Ads initialization in MainApplication (with home-screen readiness state), reads REVENUECAT_API_KEY and optional AdMob IDs from root local.properties into BuildConfig, and provides a home screen that navigates to per-format routes (banner, interstitial, app open, rewarded, native, diagnostics). Format screens are placeholders for follow-up work; shared UI (AdScreen Direct/Preloaded chips, StatusCard, ActionRow) and a native ad layout are included for later layers. The README documents local run steps and the intended tracking coverage.

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[RevenueCat-Danger-Bot]

	1 Message
📖	This PR changes 313 lines of production Kotlin/Java (limit 300); skipped via skip-pr-lines-changed-check label.

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