fix: add android r8 rules

[ovitrif]

Fixes #23

The Android AAR now ships targeted R8 consumer keep rules for the UniFFI/JNA FFI surface.

Consuming apps can enable R8 full mode without extra keep rules for this library.

Description

• Packages bindings/android/consumer-rules.pro into the AAR as proguard.txt.
• Keeps JNA Structure fields and constructors, Callback methods, and UniffiLib / IntegrityCheckingUniffiLib native method names in com.synonym.vssclient.
• Fails bundleReleaseAar when those consumer keep rules are missing or empty.

Preview

N/A — Android library packaging only.

QA Notes

Local verification: ./gradlew :validateConsumerKeepRules from bindings/android.

This repo does not keep generated Android jniLibs in git. The AAR proguard.txt check is in Gradle and runs on release builds that have those artifacts.