render: real-world CSS rendering parity

Colors: oklch()/oklab() and color-mix(); linear-gradient backgrounds.
Grid: named-line placement, repeat() expansion, drop whitespace-only
grid items; upgrade taffy 0.7 -> 0.12 so aspect-ratio images in
auto-height rows no longer collapse to slivers.
CSS: @layer, nesting, @property, :is()/:where(), clamp(), @media not all;
source/track/param/area and svg desc/metadata as display:none.
Images: webp + intrinsic sizing, srcset/picture, browser UA + Accept for
CDN fetches, grey placeholder on failure; width/height HTML attributes
rank below author CSS with aspect-ratio preserved for auto axes.

crates/obscura-cli/src/main.rs

@@ -659,7 +659,11 @@ async fn run_fetch(
         #[cfg(feature = "render")]
         {
             // Default CSS-pixel viewport, matching the engine's innerWidth/Height.
-            match page.screenshot((1280.0, 720.0)) {
+            // OBSCURA_SHOT_W / OBSCURA_SHOT_H override it (e.g. a tall viewport to
+            // capture below-the-fold content in one shot).
+            let shot_w = std::env::var("OBSCURA_SHOT_W").ok().and_then(|v| v.parse::<f32>().ok()).unwrap_or(1280.0);
+            let shot_h = std::env::var("OBSCURA_SHOT_H").ok().and_then(|v| v.parse::<f32>().ok()).unwrap_or(720.0);
+            match page.screenshot((shot_w, shot_h)) {
                 Some(bytes) => std::fs::write(path, &bytes)?,
                 None => anyhow::bail!("screenshot failed: page has no DOM to render"),
             }
@@ -1819,6 +1823,7 @@ mod tests {
             quiet: true,
             output: None,
             storage_dir: None,
+            screenshot: None,
         });
         assert!(is_quiet_command(&cmd));
     }