feat(testrun): refuse a run that dispatched none of its actions

Same argument as the zero-property refusal: an instrument that drove
nothing must not report a clean result. A first-screen violation still
wins under --exit-on-violation, --allow-no-properties exempts the
extraction sweeps that measure reach rather than judge, and one
dispatched action is enough, so a generator quiet on some screens is
untouched.

internal/testrun/testrun.go

@@ -5,9 +5,12 @@ import (
 	"context"
 	"fmt"
 	"io"
+	"maps"
 	"os"
 	"path/filepath"
+	"slices"
 	"strconv"
+	"strings"
 	"time"
 
 	"github.com/priyanshujain/sanderling/internal/android"
@@ -261,6 +264,14 @@ func Execute(ctx context.Context, options Options, stdout io.Writer) error {
 // because it holds no verdict to report. The threshold is every step and not a
 // fraction of them: a screen that composes now and then costs a healthy android
 // run a step or two, and a check that fired on those would be red on every run.
+//
+// A run that dispatched no action at all fails on the same grounds, and the
+// threshold is zero for the same reason: a generator with nothing to offer on
+// some screens is ordinary, one with nothing to offer on every screen of a whole
+// run drove nothing. --exit-on-violation keeps precedence over it so a run that
+// found something still exits on its evidence, and the property-free opt-out
+// exempts the sweeps, whose measurement is where a generator reaches and for
+// which "nowhere on this build" is a result rather than a broken run.
 func runOutcome(options Options, summary runner.Summary) error {
 	if summary.Steps > 0 && summary.SkippedVerification == summary.Steps {
 		return VacuousRunError{Steps: summary.Steps}
@@ -268,6 +279,12 @@ func runOutcome(options Options, summary runner.Summary) error {
 	if options.ExitOnViolation && len(summary.Violations) > 0 {
 		return ViolationsError{Count: len(summary.Violations)}
 	}
+	if !options.AllowNoProperties && summary.Steps > 0 && summary.DispatchedActions == 0 {
+		return NoActionsDispatchedError{
+			Steps:          summary.Steps,
+			SkippedActions: summary.SkippedActions,
+		}
+	}
 	return nil
 }
 
@@ -335,6 +352,41 @@ func (e NoPropertiesError) Error() string {
 		e.Spec)
 }
 
+// NoActionsDispatchedError reports a run not one of whose steps drove the app.
+// Every screen it judged was the one it launched on, so its empty violation list
+// says as much about the app as a spec with no properties would: the run
+// observed, judged the same state over and over, and exercised nothing. It stays
+// untyped to the CLI's violation path like VacuousRunError, so it exits 1 as a
+// run that holds no verdict rather than 2.
+type NoActionsDispatchedError struct {
+	Steps int
+	// SkippedActions is the runner's per-reason count of actions that never
+	// reached the app, which is where the cause is: a picker with no candidate
+	// reads differently from one whose every model call failed.
+	SkippedActions map[string]int
+}
+
+func (e NoActionsDispatchedError) Error() string {
+	return fmt.Sprintf(
+		"%d step(s) ran and none of them dispatched an action: the run observed the app "+
+			"and never drove it, so it judged its launch screen over and over and its "+
+			"violation count says nothing about the rest of the app%s",
+		e.Steps, skipReasonSuffix(e.SkippedActions))
+}
+
+// skipReasonSuffix renders the skip-reason tally as a trailing clause, empty
+// when the run recorded none.
+func skipReasonSuffix(skipped map[string]int) string {
+	if len(skipped) == 0 {
+		return ""
+	}
+	reasons := make([]string, 0, len(skipped))
+	for _, reason := range slices.Sorted(maps.Keys(skipped)) {
+		reasons = append(reasons, fmt.Sprintf("%s %d", reason, skipped[reason]))
+	}
+	return ". Actions that never reached the app: " + strings.Join(reasons, ", ")
+}
+
 // bundleInputs holds the pre-driver assembly: alias map, seed, esbuild defines,
 // and the resolved spec-API/goja-runtime paths the bundler consumes.
 type bundleInputs struct {

internal/testrun/testrun_test.go

@@ -297,10 +297,11 @@ func TestBuildRunMeta_OmitsModelWhenSpecDeclaresNoLLMGenerator(t *testing.T) {
 // stays a successful run, which is what every existing invocation expects.
 func TestRunOutcome_ReportsViolationsOnlyUnderTheFlag(t *testing.T) {
 	violated := runner.Summary{
-		Steps:      7,
-		Violations: []runner.ViolationRecord{{StepIndex: 3, Properties: []string{"balanceMoves"}}},
+		Steps:             7,
+		DispatchedActions: 7,
+		Violations:        []runner.ViolationRecord{{StepIndex: 3, Properties: []string{"balanceMoves"}}},
 	}
-	clean := runner.Summary{Steps: 7}
+	clean := runner.Summary{Steps: 7, DispatchedActions: 7}
 
 	if err := runOutcome(Options{}, violated); err != nil {
 		t.Errorf("without --exit-on-violation a violated run must succeed, got %v", err)
@@ -338,12 +339,68 @@ func TestRunOutcome_ARunThatJudgedNothingIsNotASuccess(t *testing.T) {
 
 	// A screen that composes now and then costs a run steps, not its verdict. A
 	// check that fired here would turn every healthy android run red.
-	mostlyJudged := runner.Summary{Steps: 6, SkippedVerification: 5}
+	mostlyJudged := runner.Summary{Steps: 6, SkippedVerification: 5, DispatchedActions: 1}
 	if err := runOutcome(Options{}, mostlyJudged); err != nil {
 		t.Errorf("a run that judged one of its 6 steps must succeed, got %v", err)
 	}
 }
 
+// A run that dispatched no action at all observed one screen for its whole life
+// and never drove the app, so its empty violation list is what an unplugged
+// instrument reports. The provider rate-limiting a model picker is the way this
+// happens in practice: every step ends with the picker handing back nothing.
+func TestRunOutcome_ARunThatDroveNothingIsNotASuccess(t *testing.T) {
+	droveNothing := runner.Summary{
+		Steps:          200,
+		SkippedActions: map[string]int{"no_action_produced": 200},
+	}
+	err := runOutcome(Options{}, droveNothing)
+	var dead NoActionsDispatchedError
+	if !errors.As(err, &dead) {
+		t.Fatalf("a run that dispatched none of its 200 steps' actions came back %v, "+
+			"want a NoActionsDispatchedError", err)
+	}
+	if dead.Steps != 200 {
+		t.Errorf("steps: got %d, want 200", dead.Steps)
+	}
+	if !strings.Contains(dead.Error(), "no_action_produced") {
+		t.Errorf("the error never names why nothing was dispatched: %v", dead)
+	}
+
+	// One action is exploration, however little. A check that fired here would
+	// be red on any run whose screen offers the generator nothing for a while.
+	droveOnce := runner.Summary{Steps: 200, DispatchedActions: 1}
+	if err := runOutcome(Options{}, droveOnce); err != nil {
+		t.Errorf("a run that dispatched one action must succeed, got %v", err)
+	}
+
+	// The sweeps that measure what a spec extracts and where a generator reaches
+	// ask for a run that judges nothing by name, and "the generator reached
+	// nothing here" is their measurement rather than their failure.
+	if err := runOutcome(Options{AllowNoProperties: true}, droveNothing); err != nil {
+		t.Errorf("the property-free opt-out no longer carries a run through, got %v", err)
+	}
+
+	// A run cut short before it took a step never got going, which the deadline
+	// and the step count already say.
+	if err := runOutcome(Options{}, runner.Summary{}); err != nil {
+		t.Errorf("a run with no steps at all must not report as dead-on-arrival, got %v", err)
+	}
+
+	// CI reads exit 2 as "the run found the bug". A run whose first screen
+	// already violated must keep reporting that, or the found bug is downgraded
+	// to a broken harness.
+	foundOnTheFirstScreen := droveNothing
+	foundOnTheFirstScreen.Violations = []runner.ViolationRecord{
+		{StepIndex: 1, Properties: []string{"balanceNonNegative"}},
+	}
+	err = runOutcome(Options{ExitOnViolation: true}, foundOnTheFirstScreen)
+	var violations ViolationsError
+	if !errors.As(err, &violations) {
+		t.Errorf("a run that found a violation came back %v, want a ViolationsError", err)
+	}
+}
+
 // wedgedLaunchDriver never returns from Launch, standing in for a driver whose
 // device-side session is stuck.
 type wedgedLaunchDriver struct {