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/schema.js

@@ -144,6 +144,29 @@ const graphUpdateNodeChangeSchema = z.object({
   reason: z.string().min(1),
 });
 
+export const answerSupportCategory = /** @type {const} */ ({
+  relative_priority_only: "relative_priority_only",
+  conditional_tradeoff: "conditional_tradeoff",
+  uncertain: "uncertain",
+  explicit_hard_constraint: "explicit_hard_constraint",
+  other: "other",
+});
+
+export const answerResolutionGuidance = /** @type {const} */ ({
+  must_remain_unresolved: "must_remain_unresolved",
+  may_resolve: "may_resolve",
+  must_resolve: "must_resolve",
+});
+
+export const answerMeaningSchema = z
+  .object({
+    userSupportedMeaning: z.string().min(1),
+    possibleInference: z.string().nullable().optional(),
+    supportCategory: z.enum(Object.values(answerSupportCategory)),
+    resolutionGuidance: z.enum(Object.values(answerResolutionGuidance)),
+  })
+  .strict();
+
 export const selectedQuestionSchema = z
   .object({
     nodeId: z.string().min(1),
@@ -160,6 +183,7 @@ export const graphUpdateSchema = z.object({
   resolvedUnknownNodeIds: z.array(z.string()).default([]),
   affectedNodeIds: z.array(z.string()).default([]),
   selectedQuestion: selectedQuestionSchema.nullable().default(null),
+  answerMeaning: answerMeaningSchema.nullable().default(null),
 });
 
 /** @typedef {z.infer<typeof graphUpdateSchema>} GraphUpdate */