render: PNG screenshots via CLI and CDP

End-to-end rasterization behind the render feature (which now enables
obscura-render/paint):

- obscura-render: screenshot_png(dom, viewport) paints and encodes to PNG
  via tiny-skia's encode_png (no extra dep).
- obscura-browser: Page::screenshot renders the live DOM at a viewport.
- CLI: obscura fetch --screenshot out.png writes the settled page to a PNG.
- CDP: Page.captureScreenshot returns the PNG base64-encoded, so
  Puppeteer/Playwright page.screenshot() works. captureSnapshot still
  errors (it is a DOM/layer snapshot, not an image).

Verified: both paths produce correct 1280x720 RGBA PNGs with elements
laid out and colored as the CSS specifies (block stacking, backgrounds,
white page). Default build unchanged; obstacle course 33/33.

Text, borders, and images are not yet painted; the box layout and
background colors are rendered.

crates/obscura-render/src/lib.rs

@@ -22,7 +22,7 @@ pub use dom::{layout_dom, DomLayout};
 #[cfg(feature = "paint")]
 mod paint;
 #[cfg(feature = "paint")]
-pub use paint::paint_dom;
+pub use paint::{paint_dom, screenshot_png};
 
 /// An axis-aligned rectangle in CSS pixels, relative to the containing block.
 #[derive(Debug, Clone, Copy, Default, PartialEq)]