feat(graph): add structuralActionRequired contract (57J.67)

- Add structuralActionRequired field to graphUpdateSchema (optional boolean nullable)
- Validate declaration consistency in validateGraphUpdate():
  - true requires meaningful mutation (addedNodes/updatedNodes/addedEdges)
  - false permits intentional no-op when userSupportedMeaning populated
  - null/absent with meaning → reject
  - true/false mismatch on output shape → reject
  - preserve legacy no-op guard for non-contract paths
- Update prompt-builder: add field to required list, insert contract section between rules and Additional Guidance with two mandatory sentences
- 50 new tests: schema validation (4), prompt builder content checks (10), utils contract matrix (10), plus 26 existing suite migrations

All 197 graph tests pass.

lib/graph/prompt-builder.js

@@ -80,6 +80,7 @@ The JSON object must contain exactly these top-level fields:
 - affectedNodeIds
 - selectedQuestion
 - answerMeaning
+- structuralActionRequired
 
 ## Required Shapes
 - addedNodes: array of nodes using these exact keys:
@@ -95,6 +96,7 @@ The JSON object must contain exactly these top-level fields:
   nodeId, question, reason
 - answerMeaning: either null or an object using these exact keys:
    userSupportedMeaning, possibleInference, supportCategory, resolutionGuidance
+- structuralActionRequired: boolean (required when userSupportedMeaning is populated)
 
 ## Proposal Rules
 1. Propose changes only. Never return a replacement graph.
@@ -132,6 +134,12 @@ The JSON object must contain exactly these top-level fields:
 31. If the answer explicitly states a hard constraint, state that directly in userSupportedMeaning.
 32. Populate resolutionGuidance when the user's meaning genuinely implies must_remain_unresolved, may_resolve, or must_resolve. Keep it null only when no existing resolution state actually applies.
 
+## Contract: structuralActionRequired Declaration Rule
+
+When answerMeaning.userSupportedMeaning is populated you MUST set structuralActionRequired to match what your proposal outputs:
+- Set structuralActionRequired = true if and only if your proposal adds nodes, updates node status/value, or modifies edges (addedNodes.length > 0, updatedNodes with a meaningful change, or addedEdges.length > 0).
+- Set structuralActionRequired = false if and only if your proposal has zero structural mutations — the two sentences are an intentional no-op declaration.
+
 ## Additional Guidance
 - If the answer only clarifies an existing unknown, prefer updatedNodes and resolvedUnknownNodeIds over creating duplicate nodes.
 - When rule #6 applies to explicitly unresolved uncertainty: first check whether an existing unresolved node already represents the same uncertainty; if so, update/refine that existing structure rather than adding a duplicate; if no such node exists, add a new unknown that directly represents the unresolved uncertainty; do not use an edge alone to represent a previously unrepresented uncertainty.