perf: cache widget DOM to avoid re-rendering on scroll (KaTeX, inline render, block headers)

[chaoxu]

Problem

CM6 virtual rendering destroys DOM for lines leaving the viewport buffer and recreates them on scroll-back. Each recreation calls `toDOM()` / `createDOM()` on every widget in the recreated lines. Several widgets do expensive work in `createDOM()`:

Expensive widgets (re-render on every scroll in/out):

Widget	File	Cost	What it does
MathWidget	`math-render.ts:110`	HIGH — calls `renderKatex()` → `katex.render()` (5-20ms per equation)	Full KaTeX HTML generation
PdfCanvasWidget	`image-render.ts:64`	HIGH — clones canvas via `drawImage()`	Canvas pixel copy
FootnoteSectionWidget	`sidenote-render.ts:255`	MEDIUM — calls `renderDocumentFragmentToDom()` which runs KaTeX for math in footnotes	Inline rendering with math
FootnoteInlineWidget	`sidenote-render.ts:443`	MEDIUM — same `renderDocumentFragmentToDom()`	Inline rendering with math
EmbedBlockWidget (plugin-render)	`plugin-render.ts:76`	MEDIUM — calls `renderDocumentFragmentToDom()` for block titles with math	Inline rendering
FrontmatterTitleWidget	`frontmatter-state.ts:107`	LOW — calls `renderDocumentFragmentToDom()` for the title	Inline rendering
HoverPreview content	`hover-preview.ts:165,255`	MEDIUM — renders block content + KaTeX for tooltip	Full block rendering
InlineRender (math)	`inline-render.ts:124`	MEDIUM — calls `katex.renderToString()`	KaTeX string generation

Cheap widgets (no caching needed):

Widget	Cost	What it does
CrossrefWidget	Cheap — createElement + textContent	Plain text spans
ClusteredCrossrefWidget	Cheap — same	Plain text spans
MixedClusterWidget	Cheap — same	Plain text spans
ImageWidget	Cheap — createElement + set src	Browser handles image caching
PdfLoadingWidget	Cheap — createElement + text	Placeholder text
CheckboxWidget	Cheap — createElement	Single checkbox element

Fix

Cache the rendered DOM inside each expensive widget. On subsequent `toDOM()` calls, clone the cached element instead of re-rendering:

```typescript
// In RenderWidget base class or per-widget:
private _cachedDOM: HTMLElement | null = null;

createDOM(): HTMLElement {
if (this._cachedDOM) return this._cachedDOM.cloneNode(true) as HTMLElement;
const el = /* ... expensive render ... */;
this._cachedDOM = el.cloneNode(true) as HTMLElement;
return el;
}
```

Cache is invalidated when `eq()` returns false (CM6 creates a new widget instance with different props, old cache is GC'd with old instance).

Impact

For a document with 30 equations, scrolling through the entire document currently triggers ~30 KaTeX re-renders. With caching: 0. Each KaTeX render is 5-20ms, so this saves 150-600ms of jank during a full scroll-through.

Scope

1. Add DOM caching to `RenderWidget` base class (or `MacroAwareWidget`)
2. Expensive widgets get it for free via inheritance
3. Cheap widgets are unaffected (the cloneNode cost is negligible)
4. No changes to decoration building or update predicates