feat(desktop): add DOM tree navigation helpers

desktop/src/preview-agent/treeNav.test.ts

@@ -0,0 +1,17 @@
+import { describe, expect, it, beforeEach } from 'vitest'
+import { climb, descend } from './treeNav'
+
+beforeEach(() => { document.body.innerHTML = `<main><section><h1>A</h1></section></main>` })
+
+describe('tree navigation', () => {
+  it('climb returns parent element, not past body', () => {
+    const h1 = document.querySelector('h1')!
+    expect(climb(h1)?.tagName.toLowerCase()).toBe('section')
+    expect(climb(document.querySelector('main')!)).toBeNull() // body 为界
+  })
+  it('descend returns first element child', () => {
+    const section = document.querySelector('section')!
+    expect(descend(section)?.tagName.toLowerCase()).toBe('h1')
+    expect(descend(document.querySelector('h1')!)).toBeNull()
+  })
+})

desktop/src/preview-agent/treeNav.ts

@@ -0,0 +1,8 @@
+export function climb(el: Element): Element | null {
+  const p = el.parentElement
+  if (!p || p.tagName.toLowerCase() === 'body' || p.tagName.toLowerCase() === 'html') return null
+  return p
+}
+export function descend(el: Element): Element | null {
+  return el.firstElementChild
+}