Update to 4.1 as an iOS-integration snapshot

[yauch-dev]

Important

Draft on purpose: waiting for the Android SDK 4.1 release. The iOS half is done and accepted; this PR merges once the Android natives expose the matching 4.1 contract and get pinned here (crossplatform + android-sdk + the rebuilt unity-wrapper AAR). Until then the package version stays 4.1.0-dev.1 and must not be published.

What

Moves the SDK to cross-platform contract 4.1.0 and native AdaptySDK-iOS 4.1.0, as an iOS-integration snapshot: the Android SDK has no 4.1 release yet, so Android stays on the 4.0.x natives (crossplatform 4.0.2 / android-sdk 4.0.1), mirroring the Flutter plugin's cut. The version is 4.1.0-dev.1 and the changelog marks it not-for-publishing until the coordinated release.

Breaking

• UpdateAttribution → UpdateExternalAttribution (source → provider), AdaptyProfile.AppliedAttributionSources → AppliedExternalAttributionProviders — matching the native 4.1 rename, no deprecated aliases. Wire key of the profile member unchanged.
• IAdaptyEventListener gained OnReceivePromotedPurchase(AdaptyPromotedProduct) — a deliberate abstract add, no default implementation: the iOS default is silently purchase, which should not hide in a default.
• Native iOS 4.1 reads fallback file format 11 and rejects format 10 at SetFallback (adapty_code: 2006); the demo stub is updated and the migration guide tells users to re-download theirs.

Added

• Adapty.MakePromotedPurchase + AdaptyPromotedProduct (App Store promoted purchases, iOS only). Note: the pinned native completes promoted purchases itself and does not emit the event yet (AdaptyPluginDelegate keeps the default implementation behind a TODO at tag 4.1.0); the listener method starts firing only once a future SDK release moves the exact pin to a native that reports them.
• AdaptyConfiguration.Builder.AdaptyAttributionEnabled (+SetAdaptyAttributionEnabled), off and unsent by default.
• A product request writes its offer identifier nested as subscription.offer.offer_identifier through AdaptySubscriptionOfferRequest, shared by the paywall and promoted DTOs; the flat key is no longer written (natives read both, the contract documents only the nested form).
• AdaptyFlow carries ui_schema (UIBuilder 5.1 custom layouts) to the renderer as an opaque private JObject — round-trip only, not public API.
• MIGRATION-v4.0-to-v4.1.md, README pins and the changelog section (both copies, entry-set in sync).

Deliberately not in this PR

• Android 4.1 (does not exist); obfuscated_profile_id and error 1030 re-checked per AGENTS — status quo holds.
• The contract's offer-identifier union not listing type: "code" while both iOS and this SDK handle it — pre-existing, a question for the contract owners.

Testing

• JSON layer: full four-leg matrix green (249 editor + 179 × iOS/Android/iOS+KidsMode); public surface byte-identical across platforms; contract copy byte-identical to the canonical file at tag 4.1.0.
• Contract conformance pass run against 4.1.0 (extractor mapping updated); it caught the missing #if UNITY_IOS gate on the promoted product's is_family_shareable, now aligned with the paywall product.
• Simulator acceptance on iPhone 17 Pro against live backend with a StoreKit configuration: activation (fallback format 11 accepted), GetFlow calm10 + products, a purchase through the new nested offer envelope (intro offer applied, [Environment: Xcode]), profile update arriving via did_load_latest_profile (Premium: true), UpdateExternalAttribution, and full-screen flow rendering by native UIBuilder 5.1.