Add offline symbolic and parallel weight-sync plan computation.

[copybara-service]

Add offline symbolic and parallel weight-sync plan computation.

Previously, ReshardPlanner computed the entire resharding transfer schedule sequentially on the centralized controller at step 0 after all workers registered their dynamic ip:port endpoints. For large topologies and multi-hundred-billion-parameter models, this centralized calculation delayed step 0 execution.

This change introduces two complementary capabilities to eliminate step 0 planning overhead:

• Offline symbolic planning and parallel worker loading: ReshardPlanner.compute_offline_schedule and save_offline_plan compute the N-D resharding schedule ahead of time using deterministic raiden_symbolic:// endpoint placeholders and serialize per-worker ControlRequest protobufs (<unit>.pb) alongside offline_schedule.json. At runtime, load_offline_worker_plan, load_offline_worker_plans_parallel, and load_offline_schedule load the precomputed plans in parallel and bind symbolic placeholders to live ip:port endpoints in O(unique_plans). Configurable via offline_plan_path / RAIDEN_OFFLINE_PLAN_PATH and save_offline_plan_path / RAIDEN_SAVE_OFFLINE_PLAN_PATH.
• Parallel per-worker schedule computation: compute_transfer_schedule_from_metadata now accepts target_src_units and parallel_worker_planning, allowing each source worker's schedule slice to be computed independently or across a parallel worker pool (compute_schedules_in_parallel_workers) and merged via merge_worker_schedules. Configurable via parallel_worker_planning / RAIDEN_PARALLEL_WORKER_PLANNING or RAIDEN_PLANNING_MODE=parallel_worker.