Android build fails with AGP 9.x: proguard-android.txt not found

## Description

The Android build fails when using Android Gradle Plugin 9.x because `proguard-android.txt` was removed in recent AGP versions. Only `proguard-android-optimize.txt` is available.

## Error

```
Execution failed for task ':capacitor-community-native-audio:...'
> proguard-android.txt not found
```

## Fix

In `android/build.gradle`, line 33:

```diff
- proguardFiles getDefaultProguardFile('proguard-android.txt'), 'proguard-rules.pro'
+ proguardFiles getDefaultProguardFile('proguard-android-optimize.txt'), 'proguard-rules.pro'
```

This is backwards-compatible — `proguard-android-optimize.txt` has been available since early Android SDK versions.

## Environment

- @capacitor-community/native-audio: 8.0.0
- Android Gradle Plugin: 9.2.1
- Gradle: 9.4.1
- Capacitor: 8.x

Start in android/build.gradle at line 33 and inspect the proguardFiles configuration described in the issue. Apply the requested Android Gradle Plugin compatibility change, then run the Android build to confirm the missing-file error is resolved.

Written by the indexing model from the issue text.