refactor(trace): rename Step.Action to Step.NextAction

The trace step's action field is the action chosen FOR THE NEXT iteration
based on observing this step's hierarchy, not the action that produced
this step. Rename Step.Action to Step.NextAction and the JSON tag to
next_action to make causality explicit at the data level.

internal/trace/writer.go

@@ -17,7 +17,8 @@ type Step struct {
 	Timestamp         time.Time                  `json:"timestamp"`
 	Screen            string                     `json:"screen,omitempty"`
 	Snapshots         map[string]json.RawMessage `json:"snapshots,omitempty"`
-	Action            *Action                    `json:"action,omitempty"`
+	// NextAction is the action chosen for the next iteration based on observing this step.
+	NextAction        *Action                    `json:"next_action,omitempty"`
 	Exceptions        []Exception                `json:"exceptions,omitempty"`
 	Violations        []string                   `json:"violations,omitempty"`
 	Hierarchy         *hierarchy.Tree            `json:"hierarchy,omitempty"`