优化 Mermaid 多图文档的滚动性能

[bifrost-proxy]

变更

• Mermaid 图表接近可视区时才在空闲时段开始渲染
• 图表离开可视区后卸载 SVG DOM，并保留原高度避免滚动跳动
• 再次进入可视区时直接恢复缓存结果，不重复执行 Mermaid
• 增加 content-visibility 浏览器布局/绘制优化
• 发布版本同步至 1.2.7

验证

• npm test：197 个测试文件通过，2403 个测试通过，7 个跳过
• npm run lint
• npm run renderer:build
• node scripts/sync-tauri-version.mjs --check 1.2.7
• Cargo locked metadata 校验

[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Reset the widget when CodeMirror remounts it

When a diagram scrolls far enough outside CodeMirror's rendered viewport, CodeMirror destroys its DOM while the decoration can retain the same MermaidDiagramWidget instance and call toDOM() again when the user scrolls back. Permanently setting destroyed here means the new DOM's observeVisibility() immediately returns, leaving the diagram stuck on “Rendering Mermaid diagram...” instead of restoring its cached SVG.

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