refactor(core): derive Copy for DrawMode; drop redundant .clone() calls (M-18)

DrawMode is a fieldless two-variant enum — it is trivially bitwise-
copyable. Adding Copy + updating choose_winnable_seed to take the value
directly eliminates 13 superfluous .clone() calls across solitaire_core,
solitaire_engine, solitaire_assetgen, and solitaire_wasm.

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

solitaire_engine/src/pause_plugin.rs

@@ -235,7 +235,7 @@ fn toggle_pause(
         // Snapshot current level and streak at pause time.
         let level = progress.as_deref().map(|p| p.0.level);
         let streak = stats.as_deref().map(|s| s.0.win_streak_current);
-        let draw_mode = settings.as_deref().map(|s| s.0.draw_mode.clone());
+        let draw_mode = settings.as_deref().map(|s| s.0.draw_mode);
         spawn_pause_screen(
             &mut commands,
             level,