Binding Proposal - fmsg - federated durable threads transport

This proposal defines an A2A 1.0 binding over fmsg, a federated, durable, store-and-forward messaging protocol. It carries A2A requests, responses, streaming events, and attachments between addresses such as `@agent@example.com` while preserving A2A operations and data semantics.

Unlike connection-oriented HTTP and gRPC bindings, fmsg supports temporarily offline agents, cross-domain delivery, authenticated sender addresses, and cryptographically linked message histories. The current draft is:

[FMSG-004 v0.2.0](https://fmsg.org/standards/fmsg-004-a2a-binding/)

We are seeking sponsorship to incubate it as experimental-cpb-fmsg.