Structural rationale: alktty's 5-byte header is a sub-demux key for
five logical streams sharing one BiStream; a tunnel has exactly one
data stream per direction, so no type byte. TCP needs no length
prefix (channels already length-prefixes every chunk). UDP needs
boundary re-framing only, and u16 suffices (UDP max payload 65507).
Wire overhead table: tunnel TCP = 8 B/chunk total (channels header
only), tunnel UDP = 10 B/datagram, vs alktty's 13 B. udpgw
calibration: 5 B + per-datagram address.
Trade-offs accepted: no in-band control path for TCP channels ever
(post-ADR-049 this is clean; escape hatch = new ALPN, not a wire
change); sentinel collision does not exist (UDP len=0 = empty
datagram, legal; EOF is the channels-level sentinel on the BiStream
at a different layer); future multi-endpoint UDP gateway framing
lives inside datagram payloads (udpgw precedent).
Residual for the Phase 1 codec ADR: empty-datagram semantics; u16 BE
over varint (leaned, MTU-bounded anyway).
glm-5.3-flash
20672be48c