MissionOrchestrator R6 class for managing multiple concurrent Missions. Implements the Coordinator layer from Symphony's architecture: poll loop, concurrency slots, stall detection, and result aggregation.
Concurrency model:
Synchronous batch: run_all() executes up to max_concurrent missions simultaneously using parallel::mclapply (fork-based on Unix, sequential on Windows).
Async per-mission: run_async() launches a mission in a callr background process and returns a handle for polling.