Per-chart export: copy/download spec and PNG/SVG from the preview header

docs/architecture/05-rendering-theming-preview.md

@@ -101,6 +101,20 @@ async function rerender(node: HTMLElement, spec: TopLevelSpec, config: Config) {
 
 - **Do** pass `actions: false`. Astrolabe owns its own export/copy affordances;
   the library's overlay menu does not belong on the preview.
+- **The per-chart export goes through the handle, not the raw view.** `RenderHandle`
+  exposes `toImageURL('png' | 'svg', { scale, background })` so the preview's Export
+  control can rasterize/serialize the live chart without any component importing
+  `vega-embed` or touching the `View` directly — the embedding boundary holds. PNG goes
+  via `view.toCanvas` → `blob:` URL (revoked after the download); SVG via `view.toSVG`
+  → `data:` URL. Renderer-agnostic: both work from the SVG-backed LivePreview view, since
+  Vega draws to its own off-screen surface here. Two non-obvious details live in the
+  handle, not the caller: **(1) dpr-aware scale** — the PNG is drawn at
+  `scale × devicePixelRatio`, so a `1×` export is as crisp as the chart on a Retina
+  screen (raw `toImageURL` scaleFactor ignores dpr, so a naive 1× looks half-resolution
+  on a 2× display). **(2) background fill** — the chart config renders a transparent
+  background (so the on-screen chart shows the pane colour), which would make a naive
+  export transparent; an opaque colour is composited under the PNG canvas and added as a
+  full-bleed `<rect>` to the SVG. The spec-text exports (copy / `.vl.json`) need no view.
 - **SVG is the default renderer, canvas is an opt-in for many-mark previews.** SVG is
   crisp/inspectable/copyable and stays the default for the editor's LivePreview. But an
   SVG chart renders one DOM node per mark, so a many-mark chart (e.g. the Chart Builder's