The basic runtime for MACP

[ajit-zer07]

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PR Title

Implement MACP v0.1 Minimal Coordination Runtime

Summary

Introduces the core gRPC-based runtime for the Multi-Agent Coordination Protocol (MACP). This is the foundational implementation that provides session-based
message coordination between agents using a pluggable Mode system.

What's included

• gRPC service layer — MACPService with SendMessage and GetSession RPCs, defined in proto/macp.proto and served via Tonic on 127.0.0.1:50051
• Runtime kernel (src/runtime.rs) — Orchestrates validate → log → mode dispatch → apply response, serving as the central coordination engine
• Pluggable Mode system (src/mode/) — A Mode trait with ModeResponse enum enabling extensible coordination strategies:
  • DecisionMode — Simple resolve-on-keyword mode
  • MultiRoundMode — Participant-based convergence with all_equal strategy
• Session management — Thread-safe SessionRegistry with configurable TTL (default 60s, max 24h), lazy expiration, and state transitions (Open → Resolved/Expired)
• Append-only log store (src/log_store.rs) — Per-session event logging with Incoming and Internal entry types
• Comprehensive error handling (src/error.rs) — MacpError enum covering all validation and state errors
• Demo/test clients — Three binary clients exercising happy paths, error paths, and multi-round convergence
• Documentation — Architecture guide, protocol spec, and usage examples in docs/

Architecture highlights

• The runtime provides "physics" (session invariants, logging, TTL, routing) while Modes provide coordination logic
• Log-before-mutate ordering ensures auditability
• Write lock held for entire message processing (noted as a future optimization opportunity)
• No background cleanup yet — expired sessions remain in memory until restart

Test plan

• cargo build compiles successfully
• cargo test — all unit tests pass
• cargo clippy — no warnings
• cargo run starts the gRPC server on :50051
• cargo run --bin client — basic decision mode demo succeeds
• cargo run --bin fuzz_client — error path coverage passes
• cargo run --bin multi_round_client — multi-round convergence demo succeeds

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