feat(trace): emit extractor_changes per step

Add ExtractorChanges to trace.Step and a runner helper that converts
the verifier's diff map into the trace shape. The inspect UI keys
its violation breadcrumbs off this field.

internal/runner/runner.go

@@ -195,6 +195,7 @@ func Run(ctx context.Context, options Options) (Summary, error) {
 			Hierarchy:        tree,
 			Residuals:        residuals,
 			Metrics:          metrics,
+			ExtractorChanges: encodeExtractorChanges(options.Verifier.ChangedExtractors()),
 		}
 		if err := options.TraceWriter.WriteStep(step); err != nil {
 			return summary, fmt.Errorf("step %d trace: %w", stepIndex, err)
@@ -554,6 +555,20 @@ func captureScreenshot(ctx context.Context, options Options, logger *slog.Logger
 	}
 }
 
+func encodeExtractorChanges(changes map[string]verifier.ExtractorChange) map[string]trace.ExtractorChange {
+	if len(changes) == 0 {
+		return nil
+	}
+	out := make(map[string]trace.ExtractorChange, len(changes))
+	for name, change := range changes {
+		out[name] = trace.ExtractorChange{
+			Prev: json.RawMessage(change.Prev),
+			Curr: json.RawMessage(change.Curr),
+		}
+	}
+	return out
+}
+
 func encodeResiduals(residuals map[string]ltl.Formula) (map[string]json.RawMessage, error) {
 	if len(residuals) == 0 {
 		return nil, nil

internal/trace/writer.go

@@ -23,6 +23,16 @@ type Step struct {
 	Hierarchy         *hierarchy.Tree            `json:"hierarchy,omitempty"`
 	Residuals         map[string]json.RawMessage `json:"residuals,omitempty"`
 	Metrics           *Metrics                   `json:"metrics,omitempty"`
+	ExtractorChanges  map[string]ExtractorChange `json:"extractor_changes,omitempty"`
+}
+
+// ExtractorChange records the prev/curr JSON values of an extractor whose
+// observation differed between two consecutive steps. Surfaced under
+// violation rows in the inspect UI as a "what changed at this step"
+// breadcrumb.
+type ExtractorChange struct {
+	Prev json.RawMessage `json:"prev"`
+	Curr json.RawMessage `json:"curr"`
 }
 
 type Metrics struct {