feat(iframe): 添加父窗口与iframe间的页面跳转API

暴露OCDIframeAPI接口，包含getFirstVisiblePageNo和gotoPage方法
移除demo.html中重复实现的页面跳转逻辑，改用新API

demo.html

@@ -148,7 +148,14 @@
           actions.className = 'bookmark-actions'
           const jumpBtn = document.createElement('button')
           jumpBtn.textContent = '跳转'
-          jumpBtn.onclick = () => navigateToPage(item.page)
+          jumpBtn.onclick = () => {
+            const api = editorFrame.contentWindow && editorFrame.contentWindow.OCDIframeAPI
+            if (api && typeof api.gotoPage === 'function') {
+              api.gotoPage(item.page)
+            } else {
+              alert('子页面未提供跳转 API')
+            }
+          }
           const editBtn = document.createElement('button')
           editBtn.textContent = '编辑'
           editBtn.onclick = () => {
@@ -185,52 +192,20 @@
 
       addBookmarkBtn.onclick = () => {
         const name = getNextBookmarkName()
-        const page = getFirstVisiblePageNo()
+        let page = 1
+        try {
+          const api = editorFrame.contentWindow && editorFrame.contentWindow.OCDIframeAPI
+          if (api && typeof api.getFirstVisiblePageNo === 'function') {
+            page = api.getFirstVisiblePageNo()
+          } else {
+            console.warn('[demo] 子页面未暴露 getFirstVisiblePageNo，默认使用第1页')
+          }
+        } catch (e) {
+          console.warn('[demo] 获取当前可见页失败，默认第1页', e)
+        }
         addBookmark(name, page)
       }
 
-      function getFirstVisiblePageNo() {
-        const doc = editorFrame.contentDocument || editorFrame.contentWindow?.document
-        if (!doc) return 1
-        const iframeRect = editorFrame.getBoundingClientRect()
-        const canvases = Array.from(doc.querySelectorAll('canvas[data-index]'))
-        if (!canvases.length) return 1
-        const visible = canvases
-          .map(c => {
-            const rect = c.getBoundingClientRect()
-            const overlapY = Math.min(rect.bottom, iframeRect.bottom) - Math.max(rect.top, iframeRect.top)
-            return { c, rect, overlapY }
-          })
-          .filter(x => x.overlapY > 0)
-          .sort((a, b) => a.rect.top - b.rect.top)
-
-        let chosen = visible.length ? visible[0].c : null
-        if (!chosen) {
-          chosen = canvases.sort((a, b) => a.getBoundingClientRect().top - b.getBoundingClientRect().top)[0]
-        }
-        const idxStr = chosen?.getAttribute('data-index') || '0'
-        const idx = parseInt(idxStr, 10)
-        return Number.isFinite(idx) ? idx + 1 : 1
-      }
-
-      function navigateToPage(page) {
-        const doc = editorFrame.contentDocument || editorFrame.contentWindow?.document
-        if (!doc) return
-        const target0 = doc.querySelector(`canvas[data-index="${page - 1}"]`)
-        const target1 = doc.querySelector(`canvas[data-index="${page}"]`)
-        const canvas = target0 || target1
-        if (!canvas) {
-          alert(`未找到第${page}页`)
-          return
-        }
-        try {
-          canvas.scrollIntoView({ behavior: 'smooth', block: 'center' })
-          const evt = new MouseEvent('mousedown', { bubbles: true })
-          canvas.dispatchEvent(evt)
-        } catch (e) {
-          console.warn('跳转页失败', e)
-        }
-      }
 
       editorFrame.addEventListener('load', () => {
         renderBookmarks()