Add GOAWAY stream to moq-lite (version 04)

Restores GOAWAY as a bidirectional stream type (0x5) for graceful
session shutdown and migration. Either endpoint can open a Goaway
stream with an optional new session URI for the peer to reconnect to.

https://claude.ai/code/session_01LCkQcr69mLdMHo9RQSL8xG

Author: claude

draft-lcurley-moq-lite.md

@@ -178,6 +178,8 @@ There's a 1-byte STREAM_TYPE at the beginning of each stream.
 | ------- | ------------- | ---------- |
 |    0x4  | Probe        | Subscriber  |
 | ------- | ------------- | ----------- |
+|    0x5  | Goaway       | Either      |
+| ------- | ------------- | ----------- |
 
 ### Announce
 A subscriber can open a Announce Stream to discover broadcasts matching a prefix.
@@ -227,6 +229,16 @@ The publisher periodically replies with PROBE messages on the same bidirectional
 
 If the publisher does not support PROBE (e.g., congestion controller is not exposed), it MUST reset the stream.
 
+### Goaway
+Either endpoint can open a Goaway Stream (0x5) to initiate a graceful session shutdown.
+
+The sender sends a GOAWAY message containing an optional new session URI.
+If the URI is non-empty, the peer SHOULD establish a new session at the provided URI and migrate any active subscriptions.
+The peer MUST NOT open new streams on the current session after receiving a GOAWAY.
+
+The sender closes the stream (FIN) when it is ready to terminate the session.
+The peer SHOULD close all streams and the session after migrating or when it no longer needs the session.
+
 # Delivery
 The most important concept in moq-lite is how to deliver a subscription.
 QUIC can only improve the user experience if data is delivered out-of-order during congestion.
@@ -570,6 +582,20 @@ PROBE Message {
 When sent by the subscriber (stream opener): the target bitrate in bits per second that the publisher should pad up to.
 When sent by the publisher (responder): the current measured bitrate in bits per second.
 
+## GOAWAY
+A GOAWAY message is sent to initiate a graceful session shutdown with an optional redirect.
+
+~~~
+GOAWAY Message {
+  Message Length (i)
+  New Session URI (s)
+}
+~~~
+
+**New Session URI**:
+A URI for the peer to reconnect to.
+An empty string indicates no redirect; the peer should simply close the session.
+
 ## GROUP
 The GROUP message contains information about a Group, as well as a reference to the subscription being served.
 
@@ -608,6 +634,9 @@ A generic library or relay MUST NOT inspect or modify the contents unless otherw
 
 # Appendix A: Changelog
 
+## moq-lite-04
+- Added GOAWAY stream for graceful session shutdown and migration.
+
 ## moq-lite-03
 - Version negotiated via ALPN (`moq-lite-xx`) instead of SETUP messages.
 - Removed Session, SessionCompat streams and SESSION_CLIENT/SESSION_SERVER/SESSION_UPDATE messages.
@@ -648,7 +677,6 @@ A quick comparison of moq-lite and moq-transport-14:
 - No paused subscriptions (forward=0)
 
 ## Deleted Messages
-- GOAWAY
 - MAX_SUBSCRIBE_ID
 - REQUESTS_BLOCKED
 - SUBSCRIBE_ERROR