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-cdp/src/domains/page.rs

@@ -655,9 +655,10 @@ mod tests {
             !err.contains("Unknown Page method"),
             "captureScreenshot must NOT fall through to the catch-all: {err}"
         );
+        #[cfg(not(feature = "render"))]
         assert!(
-            err.contains("not supported by Obscura"),
-            "error must clearly state screenshot is unsupported: {err}"
+            err.contains("requires a build with the render feature"),
+            "error must clearly state screenshot needs the render feature: {err}"
         );
         // Same for the MHTML snapshot sibling method.
         let err2 = handle("captureSnapshot", &json!({}), &mut ctx, &None)