fix(trace): an action names the generator that produced it

The setup exclusion landed for the model arm only, because only a model
pick stamped a source. A seeded run returned setup's action through the
same entry with no marker, so its denominator still counted the login
while the model arm's did not, and the two are compared.

serializeAction names setup and seeded on the wire, so both arms are
counted by one rule. An already-recorded trace names nothing and keeps
exactly the count it was reported with; unattributed_actions counts those
steps so the old denominator cannot pass as the new one. TraceVersion is
deliberately unbumped: oracle-reduction refuses a differing version, and
a bump would make all 169 recorded runs unreplayable.

internal/verifier/marshal.go

@@ -611,6 +611,10 @@ type wireAction struct {
 	ToX            int    `json:"toX"`
 	ToY            int    `json:"toY"`
 	DurationMillis int    `json:"durationMillis"`
+	// Source names the generator that produced the action: the spec's setup or
+	// the action root. Empty from the candidate enumeration, which serializes
+	// actions nothing has chosen yet.
+	Source string `json:"source"`
 }
 
 // DecodeAction turns one serialized action (the flat camelCase wire contract)
@@ -623,6 +627,15 @@ func DecodeAction(raw json.RawMessage) (Action, error) {
 	if err := json.Unmarshal(raw, &wire); err != nil {
 		return Action{}, fmt.Errorf("decode action: %w", err)
 	}
+	action, err := actionFromWire(wire)
+	if err != nil {
+		return Action{}, err
+	}
+	action.Source = wire.Source
+	return action, nil
+}
+
+func actionFromWire(wire wireAction) (Action, error) {
 	switch wire.Kind {
 	case "Tap":
 		return Action{Kind: ActionKindTap, On: wire.Selector, X: wire.X, Y: wire.Y}, nil