WebKit Features for Safari 26.4

Safari 26.4 ships 44 new features and 191 bug fixes. Highlights include CSS Grid Lanes (masonry-style layouts via `display: grid-lanes`), WebTransport for low-latency bidirectional communication over HTTP/3/QUIC, and the Keyboard Lock API for fullscreen web apps. Other additions: name-only container queries, threaded scroll-driven animations, ReadableByteStream with BYOB support, ReadableStream async iteration, math typography improvements, CSS Zoom fixes, WebAuthn PRF extension, and significant SVG and table layout bug fixes. The release also completes blocks-in-inline layout rewrite and begins a new CSS Grid layout engine.