Function tensor_span 

pub fn tensor_span(
    tensor: &str,
    offsets: &Value,
    data_start: u64,
    file_len: u64,
) -> Result<TensorSpan>

2026-09-25: Validate one tensor’s data_offsets against the file that declared it.

offsets is the raw data_offsets JSON value, data_start is 8 + header_size (where the data section begins), and file_len is the shard’s size on disk.

Rejects, in this order: a value that is not an array of two u64s, a reversed pair (end < start, the underflow), an absolute start or end that overflows u64, and a span that runs past end-of-file. Callers get a named error instead of a wrapped length.