reasoning: add proposal-level answer meaning guard

- Add answerMeaning schema with supportCategory and resolutionGuidance enums
- Add pre-mutation guard that validates proposal alignment with answerMeaning
- Update prompt builder to instruct the model on answerMeaning contract
- Add tests for schema, guard logic, parsing defaults, and regression cases A-D

lib/graph/prompt-builder.js

@@ -69,6 +69,7 @@ The JSON object must contain exactly these top-level fields:
 - resolvedUnknownNodeIds
 - affectedNodeIds
 - selectedQuestion
+- answerMeaning
 
 ## Required Shapes
 - addedNodes: array of nodes using these exact keys:
@@ -82,6 +83,8 @@ The JSON object must contain exactly these top-level fields:
 - affectedNodeIds: array of strings
 - selectedQuestion: either null or an object using these exact keys:
   nodeId, question, reason
+- answerMeaning: either null or an object using these exact keys:
+   userSupportedMeaning, possibleInference, supportCategory, resolutionGuidance
 
 ## Proposal Rules
 1. Propose changes only. Never return a replacement graph.
@@ -111,6 +114,12 @@ The JSON object must contain exactly these top-level fields:
 23. Never use unknown enum values.
 24. Do not change existing IDs.
 25. Do not replace the whole graph, and do not restate unchanged graph content inside the proposal.
+26. answerMeaning.userSupportedMeaning must state only what the user's answer directly supports.
+27. Put any stronger interpretation in answerMeaning.possibleInference, not in userSupportedMeaning.
+28. If the answer is only a relative priority statement, use supportCategory=relative_priority_only and resolutionGuidance=must_remain_unresolved.
+29. If the answer is conditional or qualified, preserve that qualification explicitly in userSupportedMeaning.
+30. If the answer says the user is unsure or does not resolve the distinction, use supportCategory=uncertain and resolutionGuidance=must_remain_unresolved.
+31. If the answer explicitly states a hard constraint, use supportCategory=explicit_hard_constraint and resolutionGuidance=must_resolve.
 
 ## Additional Guidance
 - If the answer only clarifies an existing unknown, prefer updatedNodes and resolvedUnknownNodeIds over creating duplicate nodes.
@@ -120,6 +129,7 @@ The JSON object must contain exactly these top-level fields:
 - If you add a new unknown, its description must do two jobs in one sentence: what is unknown, and why resolving it matters for the case.
 - Treat selectedQuestion as a candidate only; the engine will apply deterministic information-value scoring after validation.
 - If the answer does not justify a change, return empty arrays for every category.
+- Use answerMeaning to preserve the answer's direct meaning even when the graph change remains unresolved.
 
 ## Example Constraint Reminder
 ${formatExampleAnswerBlock()}