Adapty SDK 4.0: migrate from Paywall to Flow

[yauch-dev]

Adopt the 4.0 cross-platform contract. A placement now returns an AdaptyFlow with a list of paywall variations instead of a single AdaptyPaywall.

• New models: AdaptyFlow, AdaptyFlowPaywall (+ ProductReference), AdaptyUIFlowView; AdaptyPaywall and its ViewConfiguration are removed
• API renames: GetFlow / GetFlowForDefaultAudience (locale parameter dropped), LogShowFlow, CreateFlowView / PresentFlowView / DismissFlowView; OpenWebPaywall and CreateWebPaywallUrl accept AdaptyFlowPaywall
• Flow views are single-use and destroyed on dismiss; purchases and errors no longer auto-dismiss the view
• Events: IAdaptyFlowsEventsListener replaces AdaptyPaywallsEventsListener (flow_view_* ids, did_receive_error, analytic events); new IAdaptyUISystemRequestsHandler for permission ask/answer and app review round-trips, IAdaptyUIObserverModeResolver for Observer-mode purchases and restores
• Listener interfaces follow the C# I-prefix convention (IAdaptyEventListener, IAdaptyOnboardingsEventsListener, ...); no legacy aliases are kept
• iOS native SDK 4.0.1 ships via Swift Package Manager only: declared as remoteSwiftPackage for External Dependency Manager 1.2.187+; a new editor build validator enforces iOS deployment target 15.0+
• Android native SDK and crossplatform bumped to 4.0.0, unity-wrapper AAR rebuilt against them
• iOS Kids Mode (App Store Kids Category / COPPA): the ADAPTY_KIDS_MODE scripting define enables the KidsMode SPM trait on the AdaptySDK-iOS package via a post-build step (strips IDFA / AdSupport / AppTrackingTransparency) and forces apple_idfa_collection_disabled in the runtime config
• Legacy onboarding API deprecated in favor of Flows; pre-4.0 deprecated members removed (SetFallbackPaywalls, IDFA collection aliases)
• report_transaction success response decoded as boolean (was incorrectly parsed as a profile and always surfaced a decoding error)
• Demo migrated to the Flow API: Paywalls tab renamed to Flows, dead locale input removed; the Editor Noop stub now returns a readable "not available in the Editor" error instead of a null-parse failure
• Demo fallback files are empty placeholders: real 4.0 fallbacks weigh ~177 MB and exceed GitHub's file size limit, download your own from the Dashboard before testing SetFallbacks
• Demo build settings: bundleVersion 4.0.0, iOS bundle id aligned, simulator profile fixed (arm64, Input System), Android locked to OpenGL ES 3 because Vulkan fails to initialize on emulators