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-js/src/lib.rs

@@ -10,3 +10,8 @@ pub mod markdown;
 
 pub use markdown::HTML_TO_MARKDOWN_JS;
 pub use v8_flags::set_v8_flags;
+
+// Screenshot rasterization (PNG bytes) from the render layer. Available when the
+// render feature (which enables obscura-render/paint) is compiled in.
+#[cfg(feature = "render")]
+pub use obscura_render::screenshot_png;