android: read abiFilters from reactNativeArchitectures (#30)

[gmaclennan]

Summary

• Replaces the hardcoded ndk.abiFilters 'armeabi-v7a', 'arm64-v8a', 'x86_64' at android/build.gradle:52 with the canonical RN helper that reads the reactNativeArchitectures gradle property — same convention used by react-native core, expo-modules-core, react-native-mmkv, react-native-skia, react-native-reanimated, react-native-vision-camera, op-sqlite, react-native-quick-crypto, and realm-js.
• Adds packagingOptions.excludes rules for ABIs outside the resolved set, so reactNativeArchitectures actually narrows the AAR. Empirical AAR inspection showed that defaultConfig.ndk.abiFilters on a library module does not filter prebuilt .so content from jniLibs.srcDirs — packagingOptions.excludes is what works.
• Consumers can now ship a narrower ABI set (e.g. arm64-only release builds) by setting reactNativeArchitectures in android/gradle.properties or passing -PreactNativeArchitectures=..., without forking.
• Closes Read abiFilters from consuming app's build.gradle #30.

Why two deviations from the canonical reactNativeArchitectures helper

We ship ~155 MB of prebuilt .so for a narrower ABI set than RN's default, so the standard helper alone would silently break:

• Fallback is our three prebuilt ABIs (armeabi-v7a, arm64-v8a, x86_64), not the canonical four. x86 is excluded because scripts/build-backend.ts does not fetch x86 prebuilds — defaulting to it would produce a missing-.so runtime crash.
• Requested set is intersected with the supported set. Unsupported ABIs (e.g. x86 from a stock expo prebuild gradle.properties) are dropped with a logger.warn. An empty intersection throws GradleException with a clear message.

Verified build-time win

With -PreactNativeArchitectures=arm64-v8a against example/android (clean rebuilds of :comapeo-core-react-native:bundleDebugAar):

Override	AAR size	.so payload	armeabi-v7a	arm64-v8a	x86_64
(none)	58 MB	155 MB	47 MB	52 MB	56 MB
-PreactNativeArchitectures=arm64-v8a	22 MB	52 MB	—	52 MB	—

.so payload drops to ~⅓ (matching the verification claim). Downstream, :app:mergeDebugNativeLibs confirms the comapeo contribution drops from ~155 MB to ~52 MB at app build time.

Note: the per-library merged_jni_libs intermediate (the mergeDebugJniLibFolders task output) does not shrink — AGP's merge step always copies the full jniLibs.srcDirs. The savings come at AAR-pack and app-pack time.

Test plan

Verified end-to-end against example/android after npx expo prebuild --platform android:

• Default (no -P) — stock gradle.properties declares all 4 RN ABIs; library prints comapeo-core-react-native: dropping unsupported ABIs x86 from reactNativeArchitectures; building armeabi-v7a, arm64-v8a, x86_64. and produces a 58 MB AAR with all 3 ABIs.
• -PreactNativeArchitectures=arm64-v8a — narrows cleanly with no warning, AAR drops to 22 MB and contains only arm64-v8a.
• -PreactNativeArchitectures=x86 — fails with reactNativeArchitectures=x86 has no overlap with the ABIs comapeo-core-react-native ships prebuilds for (armeabi-v7a, arm64-v8a, x86_64).
• Closure logic exercised standalone in a scratch gradle project across 6 input cases (none, narrow, mixed, all-4-RN, whitespace, fully-unsupported).
• AAR-content verification via unzip -l … | grep '\.so' for both default and arm64-only builds.
• CI Android workflow green.

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