RFC-0004: Wire protocol versioning & rolling upgrades

Versions

• Current: major=1 minor=1 (WIRE_MAJOR / WIRE_MINOR)
• Minimum supported major: 1 (WIRE_MIN_MAJOR)

Negotiation

WireMsg::ProtocolHello { from, version } carries ProtocolVersion { major, minor, min_major }. Peers select major = min(local.major, peer.major) if each major ≥ peer.min_major; else disconnect.

Rolling-upgrade rules

1. Never remove postcard fields in a minor bump — only add optional trailing fields.
2. Major bump required for: renamed enums, changed endianness, removed message variants.
3. New nodes must keep min_major ≤ oldest fleet major until drained.
4. Management API /v1/protocol exposes local version for out-of-band checks.

Compatibility matrix (1.x)