input+signrpc+lnwallet+multi: implement combined nonce support

Add CombinedNonce() and RegisterCombinedNonce() methods with full implementation
stack.

Interface and core implementation:
- input/musig2.go: Added methods to MuSig2Session and MuSig2Signer interfaces
- input/musig2_session_manager.go: MusigSessionManager implementation using
  HaveAllNonces flag for state tracking (simplified, no extra fields)
- internal/musig2v040: Stub implementations returning ErrUnsupportedMethod
- Mock implementations (MockInputSigner, MockSigner, DummySigner)

RPC layer:
- lnrpc/signrpc/signer.proto: RPC method definitions and messages
- lnrpc/signrpc/signer.yaml: REST API endpoint mappings
- Generated protobuf code (all .pb.go files)
- lnwallet/rpcwallet/rpcwallet.go: RPCKeyRing client implementation

The proto types and RPCKeyRing are added together since RPCKeyRing implements
the Signer interface and requires proto types to fulfill the contract.

For v0.4.0, these methods return ErrUnsupportedMethod. Use MuSig2Version100RC2
to access these features.

input/musig2_session_manager.go

@@ -302,3 +302,70 @@ func (m *MusigSessionManager) MuSig2RegisterNonces(sessionID MuSig2SessionID,
 
 	return session.HaveAllNonces, nil
 }
+
+// MuSig2RegisterCombinedNonce registers a pre-aggregated combined nonce for a
+// session identified by its ID. This is an alternative to MuSig2RegisterNonces
+// and is used when a coordinator has already aggregated all individual nonces
+// and wants to distribute the combined nonce to participants.
+//
+// NOTE: This method is mutually exclusive with MuSig2RegisterNonces for the
+// same session. Once this method is called, MuSig2RegisterNonces will return
+// an error if called later for the same session.
+func (m *MusigSessionManager) MuSig2RegisterCombinedNonce(
+	sessionID MuSig2SessionID,
+	combinedNonce [musig2.PubNonceSize]byte) error {
+
+	// Hold the lock during the whole operation.
+	m.sessionMtx.Lock(sessionID)
+	defer m.sessionMtx.Unlock(sessionID)
+
+	// Load the session.
+	session, ok := m.musig2Sessions.Load(sessionID)
+	if !ok {
+		return fmt.Errorf("session with ID %x not found", sessionID[:])
+	}
+
+	// Check if we already have all nonces.
+	if session.HaveAllNonces {
+		return fmt.Errorf("already have all nonces")
+	}
+
+	// Delegate to the version-specific implementation.
+	err := session.session.RegisterCombinedNonce(combinedNonce)
+	if err != nil {
+		return fmt.Errorf("error registering combined nonce: %w", err)
+	}
+
+	// Mark that we have all nonces now.
+	session.HaveAllNonces = true
+
+	return nil
+}
+
+// MuSig2GetCombinedNonce retrieves the combined nonce for a session identified
+// by its ID. This will be available after either all individual nonces have
+// been registered via MuSig2RegisterNonces, or a combined nonce has been
+// registered via MuSig2RegisterCombinedNonce.
+func (m *MusigSessionManager) MuSig2GetCombinedNonce(
+	sessionID MuSig2SessionID) ([musig2.PubNonceSize]byte, error) {
+
+	// Hold the lock during the operation.
+	m.sessionMtx.Lock(sessionID)
+	defer m.sessionMtx.Unlock(sessionID)
+
+	// Load the session.
+	session, ok := m.musig2Sessions.Load(sessionID)
+	if !ok {
+		return [musig2.PubNonceSize]byte{}, fmt.Errorf("session with "+
+			"ID %x not found", sessionID[:])
+	}
+
+	// Get the combined nonce from the session.
+	combinedNonce, err := session.session.CombinedNonce()
+	if err != nil {
+		return [musig2.PubNonceSize]byte{}, fmt.Errorf("error getting "+
+			"combined nonce: %w", err)
+	}
+
+	return combinedNonce, nil
+}