artifacts: demo scrollbar death, figure natural sizing + fullscreen, downloads, invert toggles (release polish 7)

- demo iframe: scrolling=no (the bridge auto-sizes; a scrollable iframe
  trapped the wheel) + content-box so the bridge height is the viewport
- figures: svgNaturalWidth hug + center (w-fit max-w-full mx-auto),
  full-width fallback for viewBox-only svgs
- fullscreen: MermaidOverlay figure mode (raw svg, white stage, invert
  carried); openFigure context; figureFocus rides the mermaid Esc layer
- downloads: figure.svg + <slug>.html (raw pre-bridge), shared download.ts
- invert: manual ContrastIcon toggle on both cards — invert(1)
  hue-rotate(180deg) on the white-bearing element, default off, no
  detection, no theme wiring (owner rulings 2026-10-07)

1433 tests green; live smoke passed (hug/center measured, filters verified,
downloads fired, Esc closed; wheel-chaining structurally sound)

tests/render-md-blocks.test.tsx

@@ -160,7 +160,12 @@ describe("mermaid expand — the context seam", () => {
 		// owner is chat.tsx — geometry covered by the tests above.
 		const opened: string[] = [];
 		const html = renderToStaticMarkup(
-			<SessionMermaidContext.Provider value={{ openMermaid: (chart) => void opened.push(chart) }}>
+			<SessionMermaidContext.Provider
+				value={{
+					openMermaid: (chart) => void opened.push(chart),
+					openFigure: () => {},
+				}}
+			>
 				<div />
 			</SessionMermaidContext.Provider>,
 		);