feat: add Base64 image encoding

[mrousavy]

Summary

• add Image.toBase64Async(format, quality?) with a documented standard, padded Base64 contract
• encode directly from native UIImage and Bitmap off the caller thread, without an intermediate JS ArrayBuffer
• regenerate Nitro bindings and document data URL usage and binary-data tradeoffs
• add harness coverage for output shape, byte equality, image roundtripping, defaults, quality rounding, and errors

Tests

• bun run typecheck
• bun run lint-ci
• cd packages/react-native-nitro-image && bun run build
• cd packages/react-native-nitro-image && bun run specs
• ./gradlew :react-native-nitro-image:compileDebugKotlin --no-daemon
• ./gradlew app:assembleDebug -PreactNativeArchitectures=arm64-v8a --no-daemon
• iOS NitroImage library scheme build for the iPhone 17 Pro simulator
• bun example test:harness __tests__/image-encoding.harness.ts --harnessRunner android-device on a physical Android device - 16 tests passed

Notes

The iOS library target builds successfully. The local example app link is currently blocked by framework React not found before it can run, so iOS runtime harness coverage is left to CI.

This adds a required method to HybridImageSpec. Existing iOS types conforming through NativeImage receive the default implementation.

Closes #174