feat(parser_app): verify gateway-signed payment markers

proto/parser/parser.proto

@@ -55,6 +55,25 @@ message ParseRequest {
   // Default false preserves the pre-feature behavior byte-for-byte: the parser
   // emits an empty `intermediate_output` and the signed digest is unchanged.
   bool include_intermediate_output = 4;
+
+  // Borsh-encoded SignedVerifiedPaymentMarker: a VerifiedPaymentMarker plus
+  // the gateway's P256 signature over it. Note the wrapper, not the bare
+  // VerifiedPaymentMarker: an external signer that emits the inner struct
+  // produces bytes the enclave cannot deserialize. parser_app verifies this
+  // before processing when payment enforcement is on (PaymentPolicy::Required);
+  // today every call site passes PaymentPolicy::Disabled, so verification is a
+  // no-op and this field is not yet enforced. Empty for the open v1 routes and
+  // for local-dev / gRPC-direct callers. Field 5, not 4: field 4 shipped as
+  // include_intermediate_output (#414) while payment_marker was still on an
+  // unmerged branch, so payment_marker moved rather than breaking the wire.
+  // Size bound: parser_app rejects a payment_marker over 8 KiB on length
+  // alone, before Borsh-decoding it
+  // (parser_app::payment_verify::MAX_PAYMENT_MARKER_BYTES). A real marker is
+  // well under 1 KiB, so the limit is headroom, not a constraint a signer
+  // has to plan around. The server's whole-request gRPC cap (25MB,
+  // host_primitives::GRPC_MAX_RECV_MSG_SIZE) still applies on top, as it
+  // does to every other field on this message.
+  bytes payment_marker = 5;
 }
 
 message ChainMetadata {

src/host_primitives/Cargo.toml

@@ -6,6 +6,7 @@ publish = false
 
 [dependencies]
 qos_core = { workspace = true}
+qos_crypto = { workspace = true } # sha_256 for payment_marker hashing
 
 tonic = { version = "0.10", default-features = false }
 prost = { version = "0.12", features = [

src/host_primitives/src/lib.rs

@@ -9,6 +9,7 @@ use prost::Message;
 use qos_core::protocol::{ProtocolError, msg::ProtocolMsg};
 use tonic::Status;
 
+pub mod payment_marker;
 pub mod turnkey;
 
 /// Buffer size for socket message queue.