Adds rejectedProposalSnapshot to orchestrator diagnostics for
proposal_compatibility rejections — exposing answerMeaning (userSupportedMeaning,
possibleInference), addedNodes structural fields, addedEdges structural fields,
updatedNodes summaries, and resolvedUnknownNodeIds. Diagnostic evidence only;
does not alter validation, mutation, or error messages. Stage-gated to
proposal_compatibility only.
