oa::AutogradMeta

— per-tensor autograd state (PyTorch-style nullptr-optimized). Inference tensors pay zero bytes: autograd_ stays nullptr. Invariants: - leaf ⇔ !gradFn - leaf with requiresGrad owns a persistent grad matrix (Tier 1) - non-leaf has gradFn; grad is not populated unless retainGrad (v1: unsupported)