One fold motion for collapsing on the next message

Rows that start closing in one commit, together with rows that start revealing in it, form one batch. The driver's four steps are:

• begin: measure the boxes and choose the landing;
• prepare: grow the room before box writes so the browser does not clamp an intermediate frame;
• frame: write box heights and scrollTop together;
• finish: install the final room.

Content added during the batch updates the remaining movement by starting a new segment.

A row that starts closing or revealing while a batch runs keeps its current size, without a transition of its own, until that batch settles; the next flush drives it in a new batch, or starts a held revealing row's own transition when nothing closes. Rows therefore never change outside a batch while following continues, which would follow the bottom room without the growth that should have consumed it. A batch takes over every member at its current size: a closing row shrinks from the height it has now, and a revealing row grows from the height its own transition already reached, so only the remaining growth enters the landing; a closing row with no size left finishes without animation.

A batch with only revealing rows needs no room and keeps CSS transitions. When the viewport has no reading policy attached, closing rows also keep CSS transitions, and held rows finish immediately; under reduced motion closing rows finish immediately.

Closing rows commit their hidden state first, the driver then installs the final geometry, and only then are fold-idle listeners notified (for example, clearing room after a display-mode change).

The generic path is removed: the reserve callback, the pendingReserve and pendingGrowth reservation arithmetic, reserveBelow, and the post-fold followAfterFold. reclaimBelow (returning room after new content or reader scrolling) and resetBelow (clearing room after a display-mode change) remain.

同一次提交中开始收起的行，连同同批开始展开的行，组成一个批次。驱动器的四步依次为：

• begin：测量这些盒子，选定落点；
• prepare：在写盒子之前扩大补白，避免中间帧被浏览器截断；
• frame：同步写盒子高度和 scrollTop；
• finish：安装最终补白。

批次中途新增的内容按分段规则更新剩余运动。

批次运行期间开始收起或展开的行保持当前尺寸、不走自己的过渡，等当前批次落定后由下一次刷新处理：有收起行时进入新批次，只剩展开行时启动它自己的过渡。因此不会出现「批次之外有行在变化、同时跟随照常进行」的情况；否则跟随会把底部空间整块跟进去，而本该抵扣它的新内容没有被计入。批次按每个成员的当前尺寸接管：收起行从现有高度收缩，展开行从它自己的过渡已达到的高度继续增长，只有剩余的增长量计入落点；已无尺寸的收起行直接完成、不做动画。

只含展开行的批次不需要补白，仍用 CSS 过渡。viewport 未挂载阅读策略时，收起行也用 CSS 过渡、被挂起的行立即完成；减少动态效果时收起行立即完成。

收起行先提交 hidden，驱动器再安装最终几何，之后才通知 fold-idle 监听者（例如切换展示模式后清空补白）。

通用路径已删除：reserve 回调、pendingReserve 与 pendingGrowth 的预留计算、reserveBelow、折叠结束后再跟随的 followAfterFold。保留 reclaimBelow（新内容到来或读者滚动后归还补白）与 resetBelow（切换展示模式后清空补白）。