Function synthesized_reset_code 

fn synthesized_reset_code(err: &ProxyError) -> u64

The application error code to reset a forwarded data stream with when the source read failed for a reason that is not a peer RESET_STREAM.

0x3 (SESSION_CLOSED on drafts 11-19) for a connection-level failure — literally true when the relay dies mid-subgroup, and the code a real publisher would send. 0x0 (INTERNAL_ERROR) for everything else, which is verbatim what a proxy-internal failure is. Deliberately not 0x1 CANCELLED: its text asserts a control-plane event that never happened and points the receiver at a PUBLISH_DONE that will never arrive.

The 0x3 arm is unreachable through the QUIC transport today, and that is a defect one level down, not here. moqtap-client/src/transport/quic.rs:87-92 maps quinn::ReadError with one typed arm — Reset(code) — and collapses everything else, including ReadError::ConnectionLost(_), into TransportError::Read(String). Nothing in the workspace ever constructs TransportError::ConnectionLost from a real read, so a relay that dies mid-subgroup arrives here as Read(..) and is reset with 0x0 rather than 0x3. The stream is still reset rather than FINed, which is the substance of the guarantee — a truncated group never looks complete — and only the code is less specific than it should be. Closing it is one arm in that From impl (ReadError::ConnectionLost(_) => TransportError::ConnectionLost), in a crate this one does not own.