channel_id is u32 on the wire (~4B ids, monotonic counter, ADR-040); 256 is the default per-connection max_channels (memory bound, 256x1MiB reassembly buffers) and the default per-identity DoS cap (ADR-041) — both configurable without a wire change. AGENTS.md convention 10 and the OQ-SK-03 composed-egress note now carry the qualifier.