[PoC] Checkpoints sample app

[tonidero]

Description

This is a PoC to test the gate-focused checkpoints API in a realistic integration.

Adds examples/checkpoints-sample, a minimal app shaped like a real integration rather than a feature-by-feature exerciser (that's examples/checkpointtester):

• Onboarding (2 steps): finishing runs the onboarding_complete checkpoint and continues to home once the callback fires, whatever was served. Mimics a post-onboarding soft paywall.
• Home: a "Tapper game" card, locked until any entitlement is active (loaded via awaitCustomerInfo(FETCH_CURRENT)). Tapping play while locked runs the play_game checkpoint with a games_played custom variable. The callback firing means the gate let the user through, so the game opens whatever was served: a null result (nothing presented) or a result whose grants are merged into the card. If the user backs out of the flow, the callback never fires and they stay on home with the card still locked.
• Game: dummy tap-counter screen.
• Subscriber attribute: a small floating button on every screen opens a dialog to set or unset one attribute, to exercise rules that target attributes.

The checkpoint calls are not modelled as pending operations (no spinner): the callback may legitimately never fire.

The app exercises both ways of supplying a PaywallPresenter. ParkedPaywallPresenter parks a request in a StateFlow and the app root renders a paywall while one is set; there are two instances:

• Global, registered on Purchases in MainApplication: renders SamplePaywall, a full-screen paywall (packages as selectable cards, purchase through awaitPurchase, restore, close).
• Call-site, passed through CheckpointParams.paywallPresenter by the play_game call: renders PlayGamePaywall, a bottom sheet with one purchase button per package, restore and "Not now". A "Use call-site paywall" switch on home (on by default) controls whether it is passed, so turning it off falls back to the global paywall.

In both, a purchase or a restore that leaves an active entitlement reports Purchased, closing reports Closed, and system back NavigatedBack. Only play_game resolves to an offering, so onboarding still shows the SDK's workflow.

The app reuses the paywall tester's applicationId and defaults to the checkpoint tester's API key (then PAYWALL_TESTER_API_KEY_A), so the onboarding_complete checkpoint id is shared with checkpointtester; override with CHECKPOINTS_SAMPLE_API_KEY in local.properties.

Testing

• :examples:checkpoints-sample:assembleDebug and detektAll pass.
• Manual: walk onboarding → home → play against a project with onboarding_complete / play_game checkpoints configured; with the switch on the play paywall is the bottom sheet, with it off the full-screen one. Back out of either and confirm nothing advances, close it and confirm it does.

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[tonidero]

Closing this one since we do have the checkpoint tester app already