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/home_plugin.rs

@@ -430,7 +430,7 @@ fn build_home_context<'a>(
         challenge_best: stats.map_or(0, |s| s.0.challenge_best_score),
         daily_today,
         draw_mode: settings
-            .map(|s| s.0.draw_mode.clone())
+            .map(|s| s.0.draw_mode)
             .unwrap_or(DrawMode::DrawOne),
         font_res,
         difficulty_expanded,