Function ed25519PrivToX25519

ed25519PrivToX25519(edPrivateKey: Uint8Array): Uint8Array
Converts an Ed25519 private key to its X25519 equivalent (RFC 7748 Montgomery form) — needed for identity keys, which are Ed25519 in Me2eM but must participate in ECDH.

Uses @noble/curves ed25519.utils.toMontgomerySecret() for correct clamping per RFC 7748 Section 5.
Parameters
- edPrivateKey: Uint8Array
  32-byte Ed25519 private key.
Returns Uint8Array
32-byte X25519-compatible private key.
Throws
'KEY/INVALID_LENGTH' if input ≠ 32B.
Example
```
const xPriv = ed25519PrivToX25519(edPrivKey);
const xPub = deriveX25519PublicKey(xPriv);
```
- Defined in packages/crypto/src/core/ecdh.ts:76