phon's wire contract — the most widely depended-on layer, with no engine and no language binding.
This crate owns everything portable about a phon schema: the schema model,
content-derived schema identity, the dynamic [value] type, and the
self-describing codec that bootstraps schema exchange. Every other phon crate
depends on this one; it depends on nothing phon-specific.
Spec: docs/content/spec.md — "Type system", "Schema identity",
"Self-describing mode", and r[crates.concern-separation].