feat(android): add native mobile ink support

[markm39]

Summary

• Add an Android React Native library implementation for @mathnotes/mobile-ink, backed by the shared C++ Skia engine and exposed through MobileInkCanvasView plus MobileInkModule.
• Wire Android autolinking/package contents and align Android batch export with iOS page-index/PDF behavior.
• Update docs/README for Android support and pin the example app's transitive PostCSS patch release so audits are clean.

Verification

• npm run typecheck
• npm test -- --runInBand
• npm run test:native:smoke
• npm run build
• npm run test:example:typecheck
• npm run test:example:export:ios
• npm pack --dry-run --ignore-scripts
• npm audit --audit-level=moderate
• npm audit --prefix example --audit-level=moderate
• npm ci --prefix example
• npx expo prebuild --platform android --no-install
• ./gradlew :app:assembleDebug --no-daemon
• ./gradlew :mathnotes_mobile-ink:compileDebugKotlin --no-daemon

Review gates

• /simplify: addressed the brittle Android export JSON parsing by decoding payloads in Kotlin before JNI; removed deprecated MapBuilder; added Android-local generated-output ignores.
• /security: removed noisy URI/touch debug logs from the Android port and fixed the example PostCSS audit advisory with a narrow override instead of the breaking audit-force path.

Verification gap

• I did not run a live Android emulator/device drawing interaction in this session. The Android validation here is native compilation, autolinking through the generated example app, package contents, and APK assembly.