Settle the open question: channels header is [channel_id:u32][length:u32]
(8 bytes) with opaque payload. The 9-byte alternative (including
stream_type in the channels header) is rejected — it leaks a handler
concept into the channels layer. The handler owns its framing entirely
within the payload. TTY's 5-byte format composes as payload bytes;
total header for TTY inside channels is 13 bytes (8 + 5).
glm-5.2
f03e38326c