test(runner): retire the scaffolding the package grew by copying

Four consolidations, no assertion changed and the same 217 tests passing:

harness gains run and tryRun, retiring 68 copies of context.WithTimeout plus
Run(ctx, Options{}) plus an error check. The seven sites whose error check said
something specific keep their own wording.

mock.FailurePlan retires six wrapper drivers that were the same seven-line
method with a different error constant. The three that need more than an error
on call N stay.

traceStepLine absorbs the 15 local trace-line declarations and readTraceLines is
now the only reader. No local copy disagreed with internal/trace about a JSON
tag. Three raw readers stay on purpose: one decodes a real trace.Step, one
snapshots bytes, one asserts on raw JSON text.

One embeddable base carries the five V8-path methods the five web fakes each
redeclared.

internal/runner/foreground_gate_budget_test.go

@@ -96,16 +96,11 @@ func TestRun_AppNeverReachesForegroundEndsTheRun(t *testing.T) {
 	state := newHarness(t)
 	device := &neverDrawsDriver{Driver: state.mock}
 
-	ctx, cancel := context.WithTimeout(context.Background(), 30*time.Second)
-	defer cancel()
-	summary, err := Run(ctx, Options{
-		Duration:    time.Hour,
+	summary, err := state.tryRun(t, Options{
 		IdleTimeout: time.Millisecond,
 		MaxSteps:    3,
 		BundleID:    guardedBundleID,
 		Driver:      device,
-		Verifier:    state.verifier,
-		TraceWriter: state.writer,
 	})
 
 	var notReached ForegroundNotReachedError
@@ -165,19 +160,12 @@ func TestRun_StepsOutsideTheAppAreRecordedInTheTrace(t *testing.T) {
 	state := newHarness(t)
 	device := &leavesForegroundForeverDriver{Driver: state.mock}
 
-	ctx, cancel := context.WithTimeout(context.Background(), 30*time.Second)
-	defer cancel()
-	if _, err := Run(ctx, Options{
-		Duration:    time.Hour,
+	state.run(t, Options{
 		IdleTimeout: time.Millisecond,
 		MaxSteps:    2,
 		BundleID:    guardedBundleID,
 		Driver:      device,
-		Verifier:    state.verifier,
-		TraceWriter: state.writer,
-	}); err != nil {
-		t.Fatalf("Run: %v", err)
-	}
+	})
 
 	records := preconditionRecords(t, state.writer.Directory())
 	if len(records) == 0 {