Add Panel lazy redraw cache and document widget performance plan

[Copilot]

Implements a Panel lazy redraw cache and expands the widget performance plan. Panel now caches its masked layer, reuses putimage_withalpha when clean, and uses a unified markDirty plus a short redraw window (~80 frames) to cover interaction-driven animations. Documentation keeps the broader optimization plan.

Changes Made

• Added cached masked layer for Panel and reuse via putimage_withalpha; only regenerate mask on geometry/scale changes.
• Unified dirty marking (markDirty) across size/scale/layout/event updates; maintained a limited redraw window for animations.
• Documented the redraw window rationale in code comments; performance plan doc unchanged aside from earlier additions.

Testing

• Not run (EGE/Windows-dependent environment).

Original prompt

仔细阅读Widget相关实现。现在的绘制性能不好，请理解各种逻辑，找一些解决方案，并给出方案，先不实施

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[FeJS8888]

@copilot 由于putimage_withalpha的开销小于alphafliter，所以panel应当懒标记，进行优化，对于懒标记处理可以类似Button等

[Copilot]

@copilot 由于putimage_withalpha的开销小于alphafliter，所以panel应当懒标记，进行优化，对于懒标记处理可以类似Button等

已在 docs/Widget_Performance_Plan.md 加入 Panel 懒标记与缓存复用方案，明确优先用 putimage_withalpha 重用遮罩层，必要时才用 alphafilter（66e66ed）。本次仅文档更新，无界面改动可截图。

[FeJS8888]

@copilot 依据此，先优化Panel的懒标记

[Copilot]

@copilot 依据此，先优化Panel的懒标记

已实现 Panel 懒标记与缓存：生成遮罩后的 cachedLayer，后续直接 putimage_withalpha 复用；所有尺寸/布局/事件更新统一 markDirty，交互后保持 ~80 帧自动重绘窗口以覆盖波纹等动画（ad6d2a4）。本次为渲染逻辑更新，无新增界面截图。