feat: upgrade OpenMLS 0.5 → 0.8 for security patches and GREASE support

Migrates all MLS code in quicprochat-core from OpenMLS 0.5 to 0.8:
- StorageProvider replaces OpenMlsKeyStore (keystore.rs full rewrite)
- HybridCryptoProvider updated for new OpenMlsProvider trait
- Group operations updated for new API signatures
- MLS state persistence via MemoryStorage serialization
- tls_codec 0.3 → 0.4, openmls_traits/rust_crypto 0.2 → 0.5

crates/quicprochat-core/src/identity.rs

@@ -90,7 +90,7 @@ impl IdentityKeypair {
 /// `openmls_basic_credential` crate.
 #[cfg(feature = "native")]
 impl openmls_traits::signatures::Signer for IdentityKeypair {
-    fn sign(&self, payload: &[u8]) -> Result<Vec<u8>, openmls_traits::types::Error> {
+    fn sign(&self, payload: &[u8]) -> Result<Vec<u8>, openmls_traits::signatures::SignerError> {
         let sk = self.signing_key();
         let sig: ed25519_dalek::Signature = sk.sign(payload);
         Ok(sig.to_bytes().to_vec())