docs: add Powered by OpenIAP branding to README and docs site

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: hyochan

README.md

@@ -79,6 +79,21 @@ await iap.initConnection();
 final sameIap = FlutterInappPurchase.instance; // Same instance
 ```
 
+## Powered by OpenIAP
+
+<a href="https://openiap.dev"><img src="https://github.com/hyodotdev/openiap/blob/main/logo.png" alt="OpenIAP" height="50" /></a>
+
+flutter_inapp_purchase conforms to the **[OpenIAP specification](https://openiap.dev)** — an open, vendor-neutral interoperability standard for in-app purchases. OpenIAP provides:
+
+- **Shared specification** — Common types, error codes, and purchase flows across all platforms
+- **Generated type-safe bindings** — Swift, Kotlin, Dart, and GDScript from a single GraphQL schema
+- **Platform implementations** — [openiap-apple](https://github.com/hyodotdev/openiap/tree/main/packages/apple) (StoreKit 2) and [openiap-google](https://github.com/hyodotdev/openiap/tree/main/packages/google) (Play Billing 8.x)
+- **Verification profiles** — Standardized receipt validation and purchase verification patterns
+
+Other libraries built on OpenIAP: [react-native-iap](https://github.com/hyochan/react-native-iap) · [expo-iap](https://github.com/hyochan/expo-iap) · [kmp-iap](https://github.com/hyochan/kmp-iap) · [godot-iap](https://github.com/hyochan/godot-iap)
+
+**[Learn more about the OpenIAP standard →](https://openiap.dev/docs/foundation/about)**
+
 ## Sponsors
 
 💼 **[View Our Sponsors](https://openiap.dev/sponsors)**

docs/docs/openiap.md

@@ -0,0 +1,58 @@
+---
+title: Powered by OpenIAP
+sidebar_label: OpenIAP
+---
+
+# Powered by OpenIAP
+
+<a href="https://openiap.dev">
+  <img src="https://raw.githubusercontent.com/hyodotdev/openiap/main/logo.png" alt="OpenIAP" height="60" />
+</a>
+
+flutter_inapp_purchase conforms to the **[OpenIAP specification](https://openiap.dev)** — an open, vendor-neutral interoperability standard for in-app purchases across all platforms and frameworks.
+
+## What is OpenIAP?
+
+OpenIAP is not just a library — it's a **shared specification layer** that ensures consistent, secure, and verifiable in-app purchase behavior everywhere. Instead of each framework reinventing its own types, error models, and verification flows, OpenIAP defines them once in a single GraphQL schema and generates type-safe bindings for every platform.
+
+## How It Works
+
+```
+GraphQL Schema (Single Source of Truth)
+        │
+        ▼
+   IR (Intermediate Representation)
+        │
+        ├── Swift bindings    → openiap-apple (StoreKit 2)
+        ├── Kotlin bindings   → openiap-google (Play Billing 8.x)
+        ├── Dart bindings     → flutter_inapp_purchase
+        └── GDScript bindings → godot-iap
+```
+
+## What OpenIAP Provides
+
+| Component | Description |
+|-----------|-------------|
+| **Shared Specification** | Common types, error codes, and purchase flows across all platforms |
+| **Generated Type-Safe Bindings** | Swift, Kotlin, Dart, and GDScript from a single GraphQL schema |
+| **Platform Implementations** | [openiap-apple](https://github.com/hyodotdev/openiap/tree/main/packages/apple) (StoreKit 2) and [openiap-google](https://github.com/hyodotdev/openiap/tree/main/packages/google) (Play Billing 8.x) |
+| **Verification Profiles** | Standardized receipt validation and purchase verification patterns |
+| **Conformance Tests** | Cross-platform test matrix ensuring behavioral consistency |
+
+## Libraries Built on OpenIAP
+
+| Library | Platform |
+|---------|----------|
+| [react-native-iap](https://github.com/hyochan/react-native-iap) | React Native |
+| [expo-iap](https://github.com/hyochan/expo-iap) | Expo |
+| **[flutter_inapp_purchase](https://github.com/hyochan/flutter_inapp_purchase)** | Flutter |
+| [kmp-iap](https://github.com/hyochan/kmp-iap) | Kotlin Multiplatform |
+| [godot-iap](https://github.com/hyochan/godot-iap) | Godot Engine |
+
+## Learn More
+
+- **[OpenIAP Website](https://openiap.dev)** — Full specification and documentation
+- **[OpenIAP GitHub](https://github.com/hyodotdev/openiap)** — Source code and monorepo
+- **[About OpenIAP](https://openiap.dev/docs/foundation/about)** — Project overview and vision
+- **[Governance](https://openiap.dev/docs/foundation/governance)** — Open governance model
+- **[Become a Sponsor](https://openiap.dev/docs/foundation/sponsorship)** — Support the standard

docs/sidebars.ts

@@ -76,6 +76,11 @@ const sidebars: SidebarsConfig = {
       id: "guides/ai-assistants",
       label: "AI Assistants",
     },
+    {
+      type: "doc",
+      id: "openiap",
+      label: "OpenIAP",
+    },
     {
       type: "doc",
       id: "sponsors",