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

src/main.ts

@@ -456,6 +456,69 @@ window.onload = function () {
     }
   }
 
+  // 在 iframe 场景下对父窗口暴露：当前页获取与页码跳转
+  // 说明：返回值与入参均为 1 基页码（第 1 页记为 1）
+  winAny.OCDIframeAPI = {
+    getFirstVisiblePageNo: (): number => {
+      try {
+        const container = instance.command.getContainer()
+        const canvases = Array.from(
+          container.querySelectorAll('canvas[data-index]')
+        ) as HTMLCanvasElement[]
+        if (!canvases.length) return 1
+        const viewHeight = Math.max(
+          document.documentElement.clientHeight,
+          window.innerHeight || 0
+        )
+        const visible = canvases
+          .map(c => {
+            const rect = c.getBoundingClientRect()
+            const overlapY = Math.min(rect.bottom, viewHeight) - Math.max(rect.top, 0)
+            return { c, rect, overlapY }
+          })
+          .filter(x => x.overlapY > 0)
+          .sort((a, b) => a.rect.top - b.rect.top)
+        let chosen: HTMLCanvasElement | null = visible.length ? visible[0].c : null
+        if (!chosen) {
+          chosen = canvases.sort(
+            (a, b) => a.getBoundingClientRect().top - b.getBoundingClientRect().top
+          )[0] || null
+        }
+        const idx = Number((chosen?.dataset.index ?? '0'))
+        return Number.isFinite(idx) ? idx + 1 : 1
+      } catch {
+        return 1
+      }
+    },
+    gotoPage: (pageNo: number): boolean => {
+      try {
+        if (!Number.isFinite(pageNo) || pageNo <= 0) return false
+        const index = Math.max(0, Math.floor(pageNo - 1))
+        const container = instance.command.getContainer()
+        const target = container.querySelector(
+          `canvas[data-index="${index}"]`
+        ) as HTMLCanvasElement | null
+        if (!target) return false
+        // 滚动到该页（居中）
+        target.scrollIntoView({ behavior: 'smooth', block: 'center', inline: 'nearest' })
+        // 触发一次居中位置的 mousedown，让编辑器内部更新 pageNo
+        const rect = target.getBoundingClientRect()
+        const clientX = rect.left + rect.width / 2
+        const clientY = rect.top + rect.height / 2
+        const evt = new MouseEvent('mousedown', {
+          bubbles: true,
+          cancelable: true,
+          clientX,
+          clientY
+        })
+        target.dispatchEvent(evt)
+        return true
+      } catch {
+        return false
+      }
+    }
+  }
+
   const undoDom = document.querySelector<HTMLDivElement>('.menu-item__undo')!
   undoDom.title = `撤销(${isApple ? '⌘' : 'Ctrl'}+Z)`
   undoDom.onclick = function () {