Sharded Checkpoint and Atomic Resume

A 70B-parameter training job is paused by a node failure every few hours. The checkpoint format decides whether you lose 30 minutes or 30 hours. A sharded checkpoint writes every rank's shard in parallel and records ownership in a manifest. Resume loads each rank's shard from its own file, reconstructs the state on the same world size, and the optimiser steps as if nothing happened. Atomic write keeps a half-finished checkpoint from poisoning the next resume.