feat(web): incremental render-window batch rendering engine

Add render_window.js: Python-driven incremental pagination with id+chainIndex state, anchor restore, batch guard and streaming auto-detect; wired through chat_bridge and the page shell.

ui/web/index.html

@@ -44,7 +44,7 @@
         </div>
     </div>
     
-    <!-- 聊天容器 -->
+    <!-- 聊天容器（🆕 P1-01：顶部/底部加载入口由 app.js 按需插入 chat-container 首/尾） -->
     <div id="chat-container"></div>
     
     <div id="scroll-anchor" style="height: 1px; margin-bottom: 150px;"></div>
@@ -63,7 +63,8 @@
                     onBranchSwitch: (x, d) => post("onBranchSwitch", [x, d]),
                     onDeleteMessageClicked: (x) => post("onDeleteMessageClicked", [x]),
                     onAttachmentClicked: (x) => post("onAttachmentClicked", [x]),
-                    onScrollChanged: (y, h, c) => post("onScrollChanged", [y, h, c])
+                    onScrollChanged: (y, h, c) => post("onScrollChanged", [y, h, c]),
+                    onRequestWindowPage: (s, d, b, g) => post("onRequestWindowPage", [s, d, b, g])
                 };
                 console.log("[JS] WebView2 桥接就绪，window.bridge 可用");
             } else {
@@ -76,7 +77,9 @@
         });
     </script>
     
-    <!-- 主逻辑脚本 -->
+    <!-- 🌟 主逻辑脚本 -->
+    <!-- 🆕 P1-01 渲染窗口状态机（DOM 无关；须在 app.js 之前） -->
+    <script src="render_window.js"></script>
     <script src="app.js"></script>
 </body>
 </html>