feat: implement fastn-p2p streaming API foundation for remote shell functionality

[amitu]

Summary

Implement the foundational streaming API for fastn-p2p to enable remote shell functionality. This PR adds streaming sessions alongside the existing request/response pattern, providing the foundation for rshell/rexec commands.

🎯 Implementation: fastn-p2p Streaming API

This PR implements Phase 2 of the focused plan originally outlined in PR #2202:

• Add client module with connect() function
• Implement client::Session with stdin/stdout streams
• Add basic open_uni()/accept_uni() for stderr-like streams
• Add server::Session with protocol negotiation and context integration
• Add Session -> Request conversion (into_request()) for RPC compatibility
• Preserve existing Request methods and call() API unchanged
• Basic context integration (sessions get context from fastn-context)

🔧 New Streaming API

Client Side

// Establish streaming session
let session = fastn_p2p::client::connect(our_key, target, protocol).await?;

// Main stdin/stdout streams
session.stdin   // Send to server
session.stdout  // Receive from server

// Additional streams from server
let stderr = session.accept_uni().await?; // Server -> Client unidirectional
let (recv, send) = session.accept_bi().await?; // Bidirectional

Server Side

// Session with full context and protocol access
fn handle_session(session: fastn_p2p::server::Session<Protocol>) {
    let ctx = session.context(); // fastn-context for debugging/cancellation
    let peer = session.peer();   // Client's public key
    
    // Main streams
    session.send  // Send to client (stdout)
    session.recv  // Receive from client (stdin)
    
    // Open additional streams back to client
    let stderr = session.open_uni().await?; // Server -> Client
    let (send, recv) = session.open_bi().await?; // Bidirectional
    
    // Convert to RPC pattern if needed
    let request = session.into_request(); // Full compatibility with existing handlers
}

✅ Key Features

• 🔄 Streaming Foundation: Complete client/server session types with TODO implementation stubs
• 🔗 Context Integration: Sessions integrate with fastn-context for debugging and cancellation
• 🔙 RPC Compatibility: Session::into_request() preserves existing Request-based patterns
• 📡 Multi-stream Support: Unidirectional and bidirectional streams for stderr, additional channels
• ✅ Zero Breaking Changes: All existing fastn_p2p::call() usage continues unchanged
• 📦 Modular API: Clean client:: and server:: module exports

🏗️ Implementation Status

• ✅ Type Definitions: Complete API surface with proper error types
• ✅ fastn-context Integration: Sessions have context methods and dependency
• ✅ Build Success: All code compiles without errors (TODO stubs in place)
• 🔧 Next: Fill in TODO implementations with actual iroh streaming

📊 Changes

• 7 files changed: +178 insertions, -54 deletions
• New: client::connect(), client::Session, server::Session
• Enhanced: Modular client/server modules, fastn-context dependency
• Preserved: All existing Request/Response API and call() function

This provides the complete API foundation for streaming P2P sessions needed for remote shell functionality, with clear TODO markers ready for actual iroh implementation.

🤖 Generated with Claude Code