Constant MAX_SOURCE_REF_BYTES 

pub const MAX_SOURCE_REF_BYTES: usize = _; // 1_024usize

Max encoded bytes of a node or edge source provenance reference. A source names a stream, topic, key, or id, all of which are bounded inputs already, so this is a defensive ceiling against a hostile or buggy upsert inflating per-element state. Sized at two key-lengths, since the largest variant (a key-value source) names both a namespace and a key, far under the opaque value ceiling, as a source ref is a short pointer, not a payload.