Validates an edge envelope's structure per BCR-2026-003.
An edge may be wrapped (signed) or unwrapped. The inner envelope
must have exactly three assertion predicates: 'isA', 'source',
and 'target'. No other assertions are permitted on the edge
subject. Mirrors the reference Envelope::validate_edge
Validates an edge envelope's structure per BCR-2026-003.
An edge may be wrapped (signed) or unwrapped. The inner envelope must have exactly three assertion predicates:
'isA','source', and'target'. No other assertions are permitted on the edge subject. Mirrors the referenceEnvelope::validate_edge