feat(runner): per-tick HTML capture for WebDriver-capable drivers

Type-asserts driver.WebDriver and writes <run>/html/step-NNNNN.html in
parallel with screenshot/hierarchy/metrics. Step.HTMLAvailable flips so
the inspect UI can hide the html tab on mobile runs.

internal/runner/runner.go

@@ -73,6 +73,7 @@ func Run(ctx context.Context, options Options) (Summary, error) {
 		var hierarchyErr error
 		var metrics *trace.Metrics
 		var logs []verifier.LogEntry
+		var htmlCaptured bool
 
 		g, _ := errgroup.WithContext(ctx)
 		g.Go(func() error {
@@ -89,12 +90,24 @@ func Run(ctx context.Context, options Options) (Summary, error) {
 			logs = collectLogs(ctx, options.Driver, logSince)
 			return nil
 		})
+		if web, ok := options.Driver.(driver.WebDriver); ok {
+			g.Go(func() error {
+				htmlCaptured = captureHTML(ctx, web, options.TraceWriter, logger, si, false)
+				return nil
+			})
+		}
 		if pendingPostScreenshot {
 			postStep := pendingPostScreenshotStep
 			g.Go(func() error {
 				captureScreenshot(ctx, options, logger, postStep, true)
 				return nil
 			})
+			if web, ok := options.Driver.(driver.WebDriver); ok {
+				g.Go(func() error {
+					captureHTML(ctx, web, options.TraceWriter, logger, postStep, true)
+					return nil
+				})
+			}
 			pendingPostScreenshot = false
 		}
 		g.Wait()
@@ -156,6 +169,7 @@ func Run(ctx context.Context, options Options) (Summary, error) {
 			Hierarchy:     tree,
 			Residuals:     residuals,
 			Metrics:       metrics,
+			HTMLAvailable: htmlCaptured,
 		}
 		if err := options.TraceWriter.WriteStep(step); err != nil {
 			return summary, fmt.Errorf("step %d trace: %w", stepIndex, err)
@@ -196,6 +210,9 @@ func Run(ctx context.Context, options Options) (Summary, error) {
 
 	if pendingPostScreenshot {
 		captureScreenshot(ctx, options, logger, pendingPostScreenshotStep, true)
+		if web, ok := options.Driver.(driver.WebDriver); ok {
+			captureHTML(ctx, web, options.TraceWriter, logger, pendingPostScreenshotStep, true)
+		}
 	}
 
 	summary.EndTime = time.Now()
@@ -397,6 +414,32 @@ func captureMetrics(ctx context.Context, options Options, logger *slog.Logger, s
 	}
 }
 
+// captureHTML pulls the current document HTML from a WebDriver-capable
+// driver and stamps it into the trace under html/. Returns true on a
+// successful non-empty write so the Step.HTMLAvailable flag advertises the
+// payload to the inspect UI.
+func captureHTML(ctx context.Context, web driver.WebDriver, writer *trace.Writer, logger *slog.Logger, stepIndex int, after bool) bool {
+	html, err := web.Document(ctx)
+	if err != nil {
+		logger.Warn("html capture failed", "step", stepIndex, "after", after, "err", err)
+		return false
+	}
+	if html == "" {
+		return false
+	}
+	var writeErr error
+	if after {
+		writeErr = writer.WriteHTMLAfter(stepIndex, []byte(html))
+	} else {
+		writeErr = writer.WriteHTML(stepIndex, []byte(html))
+	}
+	if writeErr != nil {
+		logger.Warn("html write failed", "step", stepIndex, "after", after, "err", writeErr)
+		return false
+	}
+	return true
+}
+
 func captureScreenshot(ctx context.Context, options Options, logger *slog.Logger, stepIndex int, after bool) {
 	image, err := options.Driver.Screenshot(ctx)
 	if err != nil {