feat: add gradient accumulation and offload checkpoint writes to a background worker

• Accumulate gradients across micro-steps: zero grad only at window start, scale loss by acc_steps, and step optimizer/scheduler at window end.
• Move snapshot serialization to a background thread so disk I/O and the “latest” copy no longer stall training; recovery checkpoints stay synchronous for crash-durable resume.
• Clone state to CPU (_to_cpu) before enqueueing to avoid GPU OOM and in-place corruption by later training updates.
• Treat rolling recovery checkpoints as a minimal resume set (LoRA+RNG only); full snapshots still store optimizer/scheduler/scaler state. Resume from a rolling checkpoint rebuilds optimizer momentum and LR schedule from scratch.
• Make CaptionProcessor RNG thread-safe with a lock for concurrent prefetch.
• Add flush()/shutdown() for deterministic test state and clean process exit.

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