feat(frontend): implement GetMempoolStream for 0-conf lightwallet txs

The CompactTxStreamer service did not implement any mempool RPC, so
lightwallets (SilentDragonXLite / ObsidianDragonLite) received UNIMPLEMENTED
from get_mempool_stream and permanently stopped their mempool monitor. As a
result the lite wallet could not see 0-confirmation transactions and only
surfaced incoming shielded chat messages after they were mined (~1 block).

Add GetMempoolStream(Empty) returns (stream RawTransaction). A single
process-wide monitor (frontend/mempool.go) polls the node's getrawmempool and
fetches each new tx via getrawtransaction (full serialized bytes, so shielded
memos are preserved -- CompactTx's 52-byte prefix would not carry a memo), then
fans the tx out to every subscribed stream. This avoids polling the node once
per connected wallet: N clients share one poll loop. The monitor starts lazily
on the first subscription, idles without touching the node when no clients are
connected, and closes all subscriber streams when a new block is mined (the
semantics the client's monitor loop expects; the client re-syncs and reconnects).

Purely additive and wire-compatible: RawTransaction field numbers and the
cash.z.wallet.sdk.rpc package match the deployed client; existing clients that
never call GetMempoolStream are unaffected.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

walletrpc/service.pb.go

@@ -694,6 +694,8 @@ type CompactTxStreamerClient interface {
 	SendTransaction(ctx context.Context, in *RawTransaction, opts ...grpc.CallOption) (*SendResponse, error)
 	// t-Address support
 	GetAddressTxids(ctx context.Context, in *TransparentAddressBlockFilter, opts ...grpc.CallOption) (CompactTxStreamer_GetAddressTxidsClient, error)
+	// Mempool
+	GetMempoolStream(ctx context.Context, in *Empty, opts ...grpc.CallOption) (CompactTxStreamer_GetMempoolStreamClient, error)
 	// Misc
 	GetLightdInfo(ctx context.Context, in *Empty, opts ...grpc.CallOption) (*LightdInfo, error)
 	GetCoinsupply(ctx context.Context, in *Empty, opts ...grpc.CallOption) (*Coinsupply, error)
@@ -807,6 +809,38 @@ func (x *compactTxStreamerGetAddressTxidsClient) Recv() (*RawTransaction, error)
 	return m, nil
 }
 
+func (c *compactTxStreamerClient) GetMempoolStream(ctx context.Context, in *Empty, opts ...grpc.CallOption) (CompactTxStreamer_GetMempoolStreamClient, error) {
+	stream, err := c.cc.NewStream(ctx, &_CompactTxStreamer_serviceDesc.Streams[2], "/cash.z.wallet.sdk.rpc.CompactTxStreamer/GetMempoolStream", opts...)
+	if err != nil {
+		return nil, err
+	}
+	x := &compactTxStreamerGetMempoolStreamClient{stream}
+	if err := x.ClientStream.SendMsg(in); err != nil {
+		return nil, err
+	}
+	if err := x.ClientStream.CloseSend(); err != nil {
+		return nil, err
+	}
+	return x, nil
+}
+
+type CompactTxStreamer_GetMempoolStreamClient interface {
+	Recv() (*RawTransaction, error)
+	grpc.ClientStream
+}
+
+type compactTxStreamerGetMempoolStreamClient struct {
+	grpc.ClientStream
+}
+
+func (x *compactTxStreamerGetMempoolStreamClient) Recv() (*RawTransaction, error) {
+	m := new(RawTransaction)
+	if err := x.ClientStream.RecvMsg(m); err != nil {
+		return nil, err
+	}
+	return m, nil
+}
+
 func (c *compactTxStreamerClient) GetLightdInfo(ctx context.Context, in *Empty, opts ...grpc.CallOption) (*LightdInfo, error) {
 	out := new(LightdInfo)
 	err := c.cc.Invoke(ctx, "/cash.z.wallet.sdk.rpc.CompactTxStreamer/GetLightdInfo", in, out, opts...)
@@ -835,6 +869,8 @@ type CompactTxStreamerServer interface {
 	SendTransaction(context.Context, *RawTransaction) (*SendResponse, error)
 	// t-Address support
 	GetAddressTxids(*TransparentAddressBlockFilter, CompactTxStreamer_GetAddressTxidsServer) error
+	// Mempool
+	GetMempoolStream(*Empty, CompactTxStreamer_GetMempoolStreamServer) error
 	// Misc
 	GetLightdInfo(context.Context, *Empty) (*LightdInfo, error)
 	GetCoinsupply(context.Context, *Empty) (*Coinsupply, error)
@@ -862,6 +898,9 @@ func (*UnimplementedCompactTxStreamerServer) SendTransaction(ctx context.Context
 func (*UnimplementedCompactTxStreamerServer) GetAddressTxids(req *TransparentAddressBlockFilter, srv CompactTxStreamer_GetAddressTxidsServer) error {
 	return status.Errorf(codes.Unimplemented, "method GetAddressTxids not implemented")
 }
+func (*UnimplementedCompactTxStreamerServer) GetMempoolStream(req *Empty, srv CompactTxStreamer_GetMempoolStreamServer) error {
+	return status.Errorf(codes.Unimplemented, "method GetMempoolStream not implemented")
+}
 func (*UnimplementedCompactTxStreamerServer) GetLightdInfo(ctx context.Context, req *Empty) (*LightdInfo, error) {
 	return nil, status.Errorf(codes.Unimplemented, "method GetLightdInfo not implemented")
 }
@@ -987,6 +1026,27 @@ func (x *compactTxStreamerGetAddressTxidsServer) Send(m *RawTransaction) error {
 	return x.ServerStream.SendMsg(m)
 }
 
+func _CompactTxStreamer_GetMempoolStream_Handler(srv interface{}, stream grpc.ServerStream) error {
+	m := new(Empty)
+	if err := stream.RecvMsg(m); err != nil {
+		return err
+	}
+	return srv.(CompactTxStreamerServer).GetMempoolStream(m, &compactTxStreamerGetMempoolStreamServer{stream})
+}
+
+type CompactTxStreamer_GetMempoolStreamServer interface {
+	Send(*RawTransaction) error
+	grpc.ServerStream
+}
+
+type compactTxStreamerGetMempoolStreamServer struct {
+	grpc.ServerStream
+}
+
+func (x *compactTxStreamerGetMempoolStreamServer) Send(m *RawTransaction) error {
+	return x.ServerStream.SendMsg(m)
+}
+
 func _CompactTxStreamer_GetLightdInfo_Handler(srv interface{}, ctx context.Context, dec func(interface{}) error, interceptor grpc.UnaryServerInterceptor) (interface{}, error) {
 	in := new(Empty)
 	if err := dec(in); err != nil {
@@ -1063,6 +1123,11 @@ var _CompactTxStreamer_serviceDesc = grpc.ServiceDesc{
 			Handler:       _CompactTxStreamer_GetAddressTxids_Handler,
 			ServerStreams: true,
 		},
+		{
+			StreamName:    "GetMempoolStream",
+			Handler:       _CompactTxStreamer_GetMempoolStream_Handler,
+			ServerStreams: true,
+		},
 	},
 	Metadata: "service.proto",
 }