fix(rusty-image): derive arValue from arString for layout match

`arValue` (height computation) and `arString` (URL `ar=` param) used
to read from different sources, so the URL could request a different
aspect than the `<img>` element reserved — caused a visible flicker
when the transformed image loaded. Both now derive from `arString`.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

src/lib/components/RustyImage.svelte

@@ -71,14 +71,18 @@
     return w / h;
   }
 
-  const arValue = $derived(parseAr(aspect));
-  const computedHeight = $derived(
-    height ?? (arValue ? Math.round(width / arValue) : undefined),
-  );
+  // Derive aspect from either the explicit `aspect` prop or the width/height pair.
+  // Both `arString` (URL `ar=` param) and `arValue` (height-derivation) must read
+  // from the SAME source — otherwise the URL requests a different shape than the
+  // `<img>` reserves and the image flickers when it loads.
   const arString = $derived(
     aspect ??
       (height && width ? `${width}/${height}` : undefined),
   );
+  const arValue = $derived(parseAr(arString));
+  const computedHeight = $derived(
+    height ?? (arValue ? Math.round(width / arValue) : undefined),
+  );
   const sizesAttr = $derived(sizes ?? `${width}px`);
 
   function baseParams(format: "avif" | "webp" | "jpeg", w: number): RustyImageTransformParams {