refactor(sacp)!: Role-based API replacing Link/Peer system

[nikomatsakis]

Summary

This PR introduces a simpler Role-based API that replaces the previous Link/Peer system. The main goals are:

• Eliminate the Jr prefix from type names (JrConnectionCx → ConnectionTo, JrResponse → SentRequest)
• Replace Link types with Role types - Instead of encoding both sides (ClientToAgent, AgentToClient), Role types encode one side (Client, Agent)
• Simplify the builder API with clearer method names (.serve() → .connect_to())
• Unify peer and role types - Agent, Client, Proxy, Conductor serve as role types, peer selectors, AND builder starters

The New API

Building a Client

use sacp::{Client, Agent, ConnectTo};

Client.connect_from()
    .name("my-client")
    .connect_with(transport, async |connection| {
        connection.send_request(InitializeRequest::new(ProtocolVersion::LATEST))
            .block_task().await?;
        Ok(())
    })
    .await

Building an Agent

use sacp::{Agent, Client, ConnectTo};

Agent.connect_from()
    .name("my-agent")
    .on_receive_request(async |req: InitializeRequest, responder, _connection| {
        responder.respond(InitializeResponse::new(req.protocol_version))
    }, sacp::on_receive_request!())
    .connect_to(transport)
    .await

Implementing ConnectTo (was Component)

use sacp::{ConnectTo, Agent, Client};

impl ConnectTo<Client> for MyAgent {
    async fn connect_to(self, client: impl ConnectTo<Agent>) -> Result<(), sacp::Error> {
        Agent.connect_from()
            .name("my-agent")
            // handlers...
            .connect_to(client)
            .await
    }
}

Key Renames

v10 (old)	v11 (new)
Component<L>	ConnectTo<R>
ClientToAgent, AgentToClient	Client, Agent
ProxyToConductor	Proxy, Conductor
JrConnectionCx<L>	ConnectionTo<R>
JrRequestCx<T>	Responder<T>
JrResponse<T>	SentRequest<T>
MessageCx	Dispatch
.serve(transport)	.connect_to(transport)
.run_until(transport, fn)	.connect_with(transport, fn)
request_cx	responder
cx	connection

Migration Guide

A comprehensive migration guide is available at md/v10_to_v11_upgrade.md with:

• Complete type and method rename tables
• 10 common upgrade patterns with before/after examples
• Conceptual explanation of Role vs Link
• Migration tips

Breaking Changes

This is a breaking change affecting all downstream code. The changes are mostly mechanical renames that can be done with search-and-replace, guided by the migration document.

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