feat(trace): add WriteHTML + Step.HTMLAvailable

Per-step HTML lives in <run>/html/step-NNNNN.html so trace.jsonl stays
line-greppable on apps with hundreds-of-KB DOMs.

internal/trace/writer.go

@@ -13,16 +13,17 @@ import (
 )
 
 type Step struct {
-	Index      int                        `json:"step"`
-	Timestamp  time.Time                  `json:"timestamp"`
-	Screen     string                     `json:"screen,omitempty"`
-	Snapshots  map[string]json.RawMessage `json:"snapshots,omitempty"`
-	Action     *Action                    `json:"action,omitempty"`
-	Exceptions []Exception                `json:"exceptions,omitempty"`
-	Violations []string                   `json:"violations,omitempty"`
-	Hierarchy  *hierarchy.Tree            `json:"hierarchy,omitempty"`
-	Residuals  map[string]json.RawMessage `json:"residuals,omitempty"`
-	Metrics    *Metrics                   `json:"metrics,omitempty"`
+	Index         int                        `json:"step"`
+	Timestamp     time.Time                  `json:"timestamp"`
+	Screen        string                     `json:"screen,omitempty"`
+	Snapshots     map[string]json.RawMessage `json:"snapshots,omitempty"`
+	Action        *Action                    `json:"action,omitempty"`
+	Exceptions    []Exception                `json:"exceptions,omitempty"`
+	Violations    []string                   `json:"violations,omitempty"`
+	Hierarchy     *hierarchy.Tree            `json:"hierarchy,omitempty"`
+	Residuals     map[string]json.RawMessage `json:"residuals,omitempty"`
+	Metrics       *Metrics                   `json:"metrics,omitempty"`
+	HTMLAvailable bool                       `json:"html_available,omitempty"`
 }
 
 type Metrics struct {
@@ -143,6 +144,30 @@ func (w *Writer) writePNG(name string, png []byte) error {
 	return os.WriteFile(filepath.Join(directory, name), png, 0o644)
 }
 
+// WriteHTML stores the page HTML for `stepIndex` under `<dir>/html/`. Empty
+// payloads are skipped so the directory only exists when there is real
+// content to render in the inspect UI.
+func (w *Writer) WriteHTML(stepIndex int, html []byte) error {
+	return w.writeHTML(fmt.Sprintf("step-%05d.html", stepIndex), html)
+}
+
+// WriteHTMLAfter writes the post-action HTML, paired with the post-action
+// screenshot.
+func (w *Writer) WriteHTMLAfter(stepIndex int, html []byte) error {
+	return w.writeHTML(fmt.Sprintf("step-%05d-after.html", stepIndex), html)
+}
+
+func (w *Writer) writeHTML(name string, html []byte) error {
+	if len(html) == 0 {
+		return nil
+	}
+	directory := filepath.Join(w.directory, "html")
+	if err := os.MkdirAll(directory, 0o755); err != nil {
+		return fmt.Errorf("mkdir html: %w", err)
+	}
+	return os.WriteFile(filepath.Join(directory, name), html, 0o644)
+}
+
 func (w *Writer) Close() error {
 	w.mutex.Lock()
 	defer w.mutex.Unlock()