ProtocolVersion

The admit-handshake protocol version this build speaks, and the range of versions it can admit.

Two peers running incompatible kuilt builds otherwise complete the handshake and then fail later, opaquely, on the first frame neither side can decode. A joiner declares its version in AdmitMessage.Hello.protocolVersion; the host compares it against MIN_SUPPORTED..MAX_SUPPORTED and rejects a mismatch at admit time with RejectCode.ProtocolMismatch (#1569).

The version is the single source of truth — never a magic number scattered across call sites. A joiner stamps CURRENT; the host gates on isSupported.

Version 2 — the star relay (#1994), and why null is no longer tolerated

Version 2 relays frames between spokes of a star fabric. Version 1 does not, so a v1 peer admitted to a v2 room would have every relayed frame black-holed — the failure the gate exists to prevent, arriving later and more opaquely than a refusal.

A peer that predates the version field sends no version, decoding as null. Before v2 that was treated as legacy and admitted. It no longer is: null means "predates #1569", i.e. definitionally incapable of relaying — exactly the population this bump exists to exclude. A version-less peer is locked out of rooms, and that is the intended cost.

The limit of this gate, stated plainly

The gate lives host-side, in SeamRoom.handleAdmitFrame. A pre-#1569 host has no gate at all: it will admit a v2 joiner and then black-hole every relayed frame. Nothing on this side can defend that case — it is documented, not fixed.