PackageDetail: add inCart state for the Compare button

When a package is already in the comparison basket, the Compare button
swaps to a "In comparison" selected-state: soft brand-50 fill +
brand-300 border + brand-700 text + leading check icon. Technically
disabled (aria-disabled + no onClick) but sx-overrides the default
greyed Mui-disabled look so it reads as "selected/added," not
"unavailable."

Pattern: e-commerce "Added to cart" state. Removal happens via the
floating CompareBar (already owns basket mutation), not this button —
keeps the responsibility split clean.

API:
- PackageDetail: new `inCart?: boolean` prop.
- PackagesStep: forwarded as `isSelectedPackageInCart?: boolean`.
- Demo route (Packages.tsx): computes `basket.has(key)` for the
  current selection and passes it through.

Storybook: new PackageDetail story `InCart` alongside the existing
`Default` and `CompareLoading` states.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/demo/apps/arrangement/routes/Packages.tsx

@@ -23,11 +23,15 @@ export function PackagesRoute() {
 
   // Compare CTA on the PackageDetail panel just adds the selection to the
   // basket. The floating CompareBar (mounted in App.tsx) handles navigation
-  // once the user has 2+ packages selected.
+  // and removal once the user has 2+ packages selected.
   const handleCompare = () => {
     if (selectedId) basket.add(makeBasketKey(provider.id, selectedId));
   };
 
+  // When the selected package is already in the basket, PackageDetail swaps
+  // the Compare button into its "In comparison" selected state.
+  const isSelectedInCart = selectedId ? basket.has(makeBasketKey(provider.id, selectedId)) : false;
+
   // Tier-3 / tier-2 providers show "nearby verified" cards instead of
   // "more from this provider".
   const secondaryList =
@@ -51,6 +55,7 @@ export function PackagesRoute() {
         )
       }
       onCompare={handleCompare}
+      isSelectedPackageInCart={isSelectedInCart}
       onNearbyPackageClick={(key) => {
         const [otherProviderId] = key.split(':');
         if (otherProviderId) navigate(`/providers/${otherProviderId}/packages`);