feat: expose derive_node_secret_from_mnemonic

[ovitrif]

Integrated in:

• refactor: remove dependency on LDK bitkit-android#596

This PR exposes a new utility function derive_node_secret_from_mnemonic that derives the node secret key from a BIP39 mnemonic phrase.

Changes

• Added derive_node_secret_from_mnemonic(mnemonic, passphrase) function
• Added InvalidMnemonic error variant
• Exposed the new function in the UniFFI bindings for Kotlin, Swift, and Python
• Bumped version to 0.7.0-rc.3

Usage

This function returns the same secret key that would be used by a Node built with the mnemonic via Builder::set_entropy_bip39_mnemonic. This is useful for applications that need access to the node secret without instantiating a full node.

let secret = derive_node_secret_from_mnemonic(
    "abandon abandon abandon...".to_string(),
    Some("optional passphrase".to_string()),
)?;

QA Notes

Testing

• cargo test passes
• cargo clippy clean
• Android bindings: ./scripts/uniffi_bindgen_generate_kotlin_android.sh
• iOS bindings: ./scripts/uniffi_bindgen_generate_swift.sh

---

Release: v0.7.0-rc.3