ToJson now throws ArgumentException when a Body key collides with a reserved
envelope field (uri/viewerId/uuid/bid/try/cat/pubSeq/playSeq); FromJson reuses
the same shared ReservedEnvelopeKeys HashSet. ReceiveNodeResultCode expanded
from 9 to 31 codes to mirror the full enums.md catalog. Two regression tests
added for the collision guard and PascalCase uri serialization.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
