experiment: add rejected proposal diagnostics to failure path

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.

lib/graph/orchestrator.js

@@ -688,6 +688,42 @@ async function updateCaseWithDependencies(body, dependencies = {}) {
     });
 
     if (!applicationResult.success) {
+      // Compact rejected-proposal snapshot for proposal_compatibility diagnostics.
+      // Diagnostic evidence only — does not alter validation or mutation.
+      const rejectedProposalSnapshot =
+        applicationResult.stage === "proposal_compatibility" && parsedProposal.proposal
+          ? {
+              answerMeaning: parsedProposal.proposal.answerMeaning
+                ? {
+                    userSupportedMeaning:
+                      parsedProposal.proposal.answerMeaning.userSupportedMeaning ?? null,
+                    possibleInference:
+                      parsedProposal.proposal.answerMeaning.possibleInference ?? null,
+                  }
+                : null,
+              updatedNodes: (parsedProposal.proposal.updatedNodes ?? []).map((n) => ({
+                nodeId: n.nodeId,
+                newValue: n.newValue,
+              })),
+              resolvedUnknownNodeIds: parsedProposal.proposal.resolvedUnknownNodeIds ?? [],
+              addedNodes: (parsedProposal.proposal.addedNodes ?? []).map((n) => ({
+                id: n.id,
+                kind: n.kind,
+                label: n.label,
+                description: n.description,
+                parentId: n.parentId ?? null,
+                dependsOn: n.dependsOn ?? [],
+                affects: n.affects ?? [],
+                childIds: n.childIds ?? [],
+              })),
+              addedEdges: (parsedProposal.proposal.addedEdges ?? []).map((e) => ({
+                fromNodeId: e.fromNodeId,
+                toNodeId: e.toNodeId,
+                relationship: e.relationship,
+              })),
+            }
+          : null;
+
       return {
         success: false,
         stage: applicationResult.stage,
@@ -774,6 +810,7 @@ async function updateCaseWithDependencies(body, dependencies = {}) {
               situationGraph.resolvedNodeIds || [],
             ),
           }),
+          ...(rejectedProposalSnapshot && { rejectedProposalSnapshot }),
         },
         statusCode:
           applicationResult.stage === "application" ||