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_assetgen/src/bin/gen_difficulty_seeds.rs

@@ -96,7 +96,7 @@ fn main() {
                 continue;
             }
             let cfg = SolverConfig { move_budget, state_budget };
-            match try_solve(seed, draw_mode.clone(), &cfg) {
+            match try_solve(seed, draw_mode, &cfg) {
                 SolverResult::Winnable => {
                     buckets[i].push(seed);
                     eprintln!(