Require mesh-llm/1 for mesh joins

[i386]

Summary

mesh joins now use only the mesh-llm/1 protobuf control-plane protocol.
This removes the legacy mesh-llm/0 JSON compatibility path from dialing, acceptance, stream handling, and protocol docs.

User Impact

Nodes running releases below v0.54.0 will no longer be able to join the mesh, because v0.54.0 was the first tagged release that contained the protobuf mesh-llm/1 control-plane implementation.
Meshes are now protobuf-only on the control plane.

What Changed

• remove mesh-llm/0 ALPN fallback and legacy JSON control-frame handling
• delete the legacy v0 protocol module and compatibility-only tests/helpers
• update protocol and testing docs to describe mesh-llm/1 as the only supported control-plane protocol

Protocol

This is an intentional compatibility break for pre-v0.54.0 nodes.
Releases below v0.54.0 cannot negotiate the protobuf control plane and will not be able to join upgraded meshes.

Validation

• just build
• cargo test -p mesh-llm --lib

[michaelneale]

I am ok with this, but need to update the web app on the main site co-ordinated with some other nodes during release - so if we can work that out?

Also - how do we kind of test this all out?

[i386]

Also - how do we kind of test this all out?

Run a pre-0.54.0 node and this change. You won't see the new node in pre-0.54.0's mesh peer list.

[ndizazzo]

LGTM! My favourite activity is deleting code