feat(assets): swap card art to hayeah/playing-cards-assets (MIT)

Replaces the previous xCards-derived card faces (LGPL-3.0) with
hayeah/playing-cards-assets, which itself derives from the
public-domain vector-playing-cards Google Code project. The whole
package is MIT now — see CREDITS.md for the new attribution table
and the simpler license summary.

solitaire_engine/assets/themes/default/
  52 face SVGs (clubs/diamonds/hearts/spades × ace/2-10/jack/queen/
  king) — copied from hayeah, renamed to the canonical
  `{suit}_{rank}.svg` form `CardKey::manifest_name` produces. The
  bundled default theme manifest references each by the same name.
  back.svg — original midnight-purple-themed card back, hand-written
  to match the project's design tokens (BG_BASE / BG_ELEVATED /
  ACCENT_PRIMARY / ACCENT_SECONDARY). MIT, original work.

assets/cards/faces/{RANK}{SUIT}.png
  52 PNGs regenerated from the new SVGs at 750-tall via resvg 0.47.
  These remain the legacy backwards-compat path that
  `card_plugin::load_card_images` reads at startup; once the runtime
  theme system finishes loading the embedded default theme, the
  CardImageSet's face handles are overwritten with the SVG-rendered
  variants and these PNGs become moot. Keeping them in place avoids
  a brief blank-card flash before the async theme load completes.

solitaire_engine/src/assets/sources.rs
  embed_default_svg!() macro + DEFAULT_THEME_SVGS table that bundles
  every face + the back into the binary at compile time via
  include_bytes!. populate_embedded_default_theme now iterates the
  table so the EmbeddedAssetRegistry is populated under the same
  asset paths the manifest references.

CREDITS.md
  License summary collapses from MIT + LGPL-3.0 + OFL-1.1 to MIT +
  OFL-1.1 (the OFL still applies to FiraMono). The hayeah upstream
  URL replaces the previously-blank xCards entry.

cargo build / clippy --workspace --all-targets -- -D warnings / test
--workspace all green (960 passed, 0 failed, 9 ignored).

solitaire_engine/assets/themes/default/back.svg

@@ -0,0 +1,40 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<!--
+  Default theme card back — Solitaire Quest's midnight-purple palette.
+  Original work, MIT-licensed alongside the rest of this project.
+  Aspect 2:3 to match the face SVGs from hayeah/playing-cards-assets.
+-->
+<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 200 300" width="200" height="300">
+  <defs>
+    <pattern id="diamonds" x="0" y="0" width="20" height="20" patternUnits="userSpaceOnUse">
+      <rect x="0" y="0" width="20" height="20" fill="#1A0F2E"/>
+      <path d="M 10 0 L 20 10 L 10 20 L 0 10 Z"
+            fill="none" stroke="#3A2580" stroke-width="1"/>
+      <circle cx="10" cy="10" r="1" fill="#FFD23F"/>
+    </pattern>
+  </defs>
+
+  <!-- Outer card surface with a midnight-purple base + diamond lattice -->
+  <rect x="0" y="0" width="200" height="300" rx="12" ry="12" fill="#1A0F2E"/>
+  <rect x="6" y="6" width="188" height="288" rx="9" ry="9" fill="url(#diamonds)"/>
+
+  <!-- Bordered inset so the lattice has a clear edge -->
+  <rect x="14" y="14" width="172" height="272" rx="6" ry="6"
+        fill="none" stroke="#FFD23F" stroke-width="1.5" opacity="0.85"/>
+
+  <!-- Centred diamond medallion -->
+  <g transform="translate(100 150)">
+    <path d="M 0 -42 L 42 0 L 0 42 L -42 0 Z" fill="#2D1B69" stroke="#FFD23F" stroke-width="2"/>
+    <path d="M 0 -22 L 22 0 L 0 22 L -22 0 Z" fill="#3A2580" stroke="#FFD23F" stroke-width="1"/>
+    <circle cx="0" cy="0" r="4" fill="#FFD23F"/>
+  </g>
+
+  <!-- Corner pips picking up the magenta secondary accent so the back
+       still reads as part of the design system at a glance -->
+  <g fill="#FF6B9D">
+    <circle cx="22" cy="22" r="2.5"/>
+    <circle cx="178" cy="22" r="2.5"/>
+    <circle cx="22" cy="278" r="2.5"/>
+    <circle cx="178" cy="278" r="2.5"/>
+  </g>
+</svg>