## Summary
- Publish OpenIAP Spec **2.1.0** in this PR and target **openiap-google
2.3.0** for the CI-promoted stable package.
- Upgrade the Google implementation to Play Billing **9.1.0** with
`BILLING_CHOICE`, developer-rendered choice metadata/dialog/reporting,
`originalExternalTransactionId`, and `showInAppMessagesAndroid`.
- Wire the complete Android surface through React Native/Nitro, Expo,
Flutter, Godot, KMP, and MAUI, including hooks, listeners, generated
types, examples, and store-specific unsupported behavior.
- Consolidate the upcoming stable release notes by platform package,
IAPKit, and framework library; update Android/API docs and durable
parity/release workflow rules.
- Clarify IAPKit purchases as verification snapshots and update examples
to verify, finish, then reverify the same token.

## Versioning
- OpenIAP Spec: **2.1.0** (directly bumped here)
- openiap-google stable target: **2.3.0** (source remains `2.3.0-rc.1`
until CI promotion)
- openiap-apple stable target: **2.2.5** (source remains `2.2.5-rc.1`
until CI promotion)
- Framework stable targets in release notes: react-native-iap 15.4.0,
expo-iap 4.4.0, flutter_inapp_purchase 9.4.0, godot-iap 2.4.0, kmp-iap
2.4.0, OpenIap.Maui 1.2.1

## Related
- Closes #221
- Addresses the verification lifecycle concern in #209
- Includes #214 in the consolidated stable release notes
- Applies the package/framework changelog grouping requested in #206

## Verification
- GQL generation and platform sync, including `yarn specs` for Nitro
- `bun audit:parity`, `bun audit:docs`, and staged diff checks, repeated
after a five-minute no-change interval
- Google Play/Horizon/Amazon compile and unit-test matrix
- Apple `swift build` and 104 Swift tests
- React Native 345 tests plus Android example build across Kotlin/C++
ABIs
- Expo 303 library tests plus 76 config-plugin tests
- Flutter analyze plus 281 tests
- KMP Play/Amazon/Horizon compiles and tests
- Godot Android/iOS artifact and example checks
- MAUI shared and Play/Amazon/Horizon builds
- IAPKit lint, 572 tests, production build, compiled-server smoke probes
- Docs typecheck, formatting, audit, and production build

## Connected-device E2E
- **Google Play / Pixel:** exercised Billing Choice availability and
`showInAppMessagesAndroid`; the test account returned
program-unavailable and `no-action-needed`, both handled correctly.
- **Fire OS:** reran app launch/catalog fetch and purchase-state reads;
the earlier branch matrix covered React Native, Expo, Flutter, and MAUI
purchase/fulfill/cancel paths.
- **Vega:** reran React Native and Expo launches; the earlier Expo flow
completed approved purchase and fulfillment.
- **Apple:** physical iPhone on iOS 26.5.2 passes build/install/launch.
Purchase UI automation is still blocked because Maestro hangs after
detecting the physical device.
- **Meta Horizon:** package/framework Horizon builds pass, but connected
purchase E2E is blocked because the Quest remains `adb unauthorized`;
the headset must accept the USB debugging prompt.

## Preview
The visible changes are API/reference documentation and native SDK
examples. Documentation production build and connected example-app
launch checks cover those surfaces.