`TensorRecord.tensor_metadata` is an opaque CBOR map. This module defines and enforces the **shape convention** used by the Python bindings: ```text { "shape": [u64, u64, ...], // tensor dimensions, row-major "dtype": "<str>" // canonical dtype name (e.g. "bf16", "mxfp4") } ``` A minimal hand-rolled CBOR encoder/decoder is used — we only need unsigned integers, text strings, fixed-length arrays, and fixed-length maps. No floats, tags, or indefinite-length items. Keeping it dependency-free avoids pulling a CBOR crate for ~50 lines of logic and guarantees the byte layout is exactly what the spec says.