design: optimize border-beam with layout containment and hardcoded path radius to eliminate corner stutter

vibn-frontend/app/(marketing)/new-site.tsx

@@ -992,6 +992,7 @@ function Hero({ onStart, variant = "quote" }) {
           padding: 1.5px;
           pointer-events: none;
           z-index: 2;
+          contain: paint;
           -webkit-mask:
             linear-gradient(#000 0 0) content-box,
             linear-gradient(#000 0 0);
@@ -1007,11 +1008,10 @@ function Hero({ onStart, variant = "quote" }) {
           position: absolute;
           width: 170px;
           aspect-ratio: 1;
-          offset-path: rect(0 auto auto 0 round var(--r-xl));
+          offset-path: rect(0 auto auto 0 round 28px);
           offset-distance: 0%;
           offset-anchor: 50% 50%;
-          /* Force GPU hardware acceleration to remove subpixel snapping
-             and corner jitter */
+          /* Force GPU hardware acceleration and prevent variable re-evals */
           will-change: transform, offset-distance;
           transform: translate3d(0, 0, 0);
           background: linear-gradient(