fix(parallax): prime parallax position with pre-Blazor init script to kill Server->WASM position pop

DeepDrftShared.Client/Interop/parallax/parallax.ts

@@ -141,6 +141,14 @@ export function register(element: HTMLElement, options: RegisterOptions): string
 
     setupAutoHeight(handle);
 
+    // Prime position synchronously so enhanced navigation and WASM handoff have
+    // zero-frame gap. The init script covers cold page load; this covers nav.
+    // Skip under reduced motion — parallax.ts never drives position then.
+    if (!reducedMotion()) {
+        element.setAttribute('data-parallax-active', '');
+        applyParallax(handle);
+    }
+
     return id;
 }