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/navigation_trace_test.go

@@ -1,17 +1,12 @@
 package runner
 
 import (
-	"bufio"
 	"context"
-	"encoding/json"
-	"os"
-	"path/filepath"
 	"testing"
 	"time"
 
 	"github.com/priyanshujain/sanderling/internal/driver"
 	mockdriver "github.com/priyanshujain/sanderling/internal/driver/mock"
-	"github.com/priyanshujain/sanderling/internal/trace"
 )
 
 // navigatingDriver reports one navigation per step, the way a page that submits
@@ -33,46 +28,20 @@ func TestRunner_TheTraceRecordsThatThePageNavigated(t *testing.T) {
 	const url = "http://127.0.0.1/index.html?"
 	navigating := &navigatingDriver{Driver: state.mock, url: url}
 
-	ctx, cancel := context.WithTimeout(context.Background(), 5*time.Second)
-	defer cancel()
-	if _, err := Run(ctx, Options{
-		Duration:    100 * time.Millisecond,
-		IdleTimeout: 20 * time.Millisecond,
-		MaxSteps:    3,
-		Driver:      navigating,
-		Verifier:    state.verifier,
-		TraceWriter: state.writer,
-	}); err != nil {
-		t.Fatalf("Run: %v", err)
-	}
+	state.run(t, Options{Duration: 100 * time.Millisecond, MaxSteps: 3, Driver: navigating})
 	if err := state.writer.Close(); err != nil {
 		t.Fatalf("close trace: %v", err)
 	}
 
-	file, err := os.Open(filepath.Join(state.writer.Directory(), "trace.jsonl"))
-	if err != nil {
-		t.Fatal(err)
-	}
-	defer file.Close()
-
 	recorded := 0
-	scanner := bufio.NewScanner(file)
-	scanner.Buffer(make([]byte, 0, 64*1024), 8*1024*1024)
-	for scanner.Scan() {
-		var step struct {
-			Index       int                `json:"step"`
-			Navigations []trace.Navigation `json:"navigations"`
-		}
-		if err := json.Unmarshal(scanner.Bytes(), &step); err != nil {
-			t.Fatalf("trace line decode: %v", err)
-		}
+	for _, step := range readTraceLines(t, state.writer.Directory()) {
 		for _, navigation := range step.Navigations {
 			recorded++
 			if navigation.URL != url {
-				t.Errorf("step %d records navigation to %q, want %q", step.Index, navigation.URL, url)
+				t.Errorf("step %d records navigation to %q, want %q", step.Step, navigation.URL, url)
 			}
 			if navigation.UnixMillis == 0 {
-				t.Errorf("step %d records a navigation with no timestamp", step.Index)
+				t.Errorf("step %d records a navigation with no timestamp", step.Step)
 			}
 		}
 	}