feat(kmp/ed25519): in-place KMP module (JNI on Android, cinterop on iOS) + mnemonic Kotlin rewrite - #1204

Node.js 20 is deprecated. The following actions target Node.js 20 but are being forced to run on Node.js 24: actions/cache@v4, actions/checkout@v4, actions/setup-java@v3, timheuer/base64-to-file@v1. For more information see: https://github.blog/changelog/2025-09-19-deprecation-of-node-20-on-github-actions-runners/