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/paint.rs

@@ -53,6 +53,12 @@ fn bg_color_for(tree: &DomTree, nid: obscura_dom::tree::NodeId) -> Option<[u8; 4
     style.background_color
 }
 
+/// Render `tree` at `viewport` to PNG bytes (RGBA 8-bit). Returns None if the
+/// viewport is zero-sized. Convenience over `paint_dom` + `encode_png`.
+pub fn screenshot_png(tree: &DomTree, viewport: (f32, f32)) -> Option<Vec<u8>> {
+    paint_dom(tree, viewport)?.encode_png().ok()
+}
+
 #[cfg(test)]
 mod tests {
     use super::*;