feat(engine): add Classic card theme

White/cream card faces with traditional red (hearts/diamonds) and black
(clubs/spades) colours, plus a navy diamond-pattern card back. Shipped
as a bundled AssetServer theme alongside the existing Default theme.

Registry updated to include the Classic entry; registry tests updated
to reflect the new BUNDLED_COUNT of 2.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

solitaire_engine/assets/themes/classic/back.svg

@@ -0,0 +1,19 @@
+<svg xmlns="http://www.w3.org/2000/svg" width="256" height="384" viewBox="0 0 256 384">
+  <defs>
+    <pattern id="dp" x="0" y="0" width="28" height="28" patternUnits="userSpaceOnUse">
+      <rect width="28" height="28" fill="#1a3a6e"/>
+      <polygon points="14,2 26,14 14,26 2,14" fill="#2255aa"/>
+      <polygon points="14,7 21,14 14,21 7,14" fill="#1a3a6e"/>
+    </pattern>
+  </defs>
+  <!-- White card background -->
+  <rect x="2" y="2" width="252" height="380" rx="14" ry="14" fill="#FAFAF8"/>
+  <!-- Red outer border -->
+  <rect x="2" y="2" width="252" height="380" rx="14" ry="14"
+        fill="none" stroke="#CC1111" stroke-width="4"/>
+  <!-- Navy diamond pattern inset -->
+  <rect x="16" y="16" width="224" height="352" rx="8" ry="8" fill="url(#dp)"/>
+  <!-- Thin red frame around pattern -->
+  <rect x="16" y="16" width="224" height="352" rx="8" ry="8"
+        fill="none" stroke="#CC1111" stroke-width="2"/>
+</svg>