feat(persistence): add high score saving/loading

Add HighScore resource, persistence module with save/load, and
integration tests. Dependencies: serde, serde_json, dirs.

tests/high_score.rs

@@ -0,0 +1,60 @@
+use bevy::ecs::world::World;
+use bglga::persistence;
+use bglga::resources::{HighScore, Score};
+use bglga::systems::check_high_score;
+
+// ── check_high_score system ──
+
+#[test]
+fn check_high_score_updates_when_score_exceeds() {
+    let save_path = std::env::temp_dir().join("bglga_test_integration.json");
+    let mut world = World::default();
+    world.insert_resource(Score { value: 7000 });
+    world.insert_resource(HighScore { value: 5000 });
+    persistence::set_save_path_override(save_path.clone());
+    world.register_system(check_high_score);
+    world
+        .run_system_cached(check_high_score)
+        .expect("system should run");
+    let hs = world.resource::<HighScore>();
+    assert_eq!(hs.value, 7000);
+    // Assert file was persisted to disk
+    let content = std::fs::read_to_string(&save_path).expect("high score file should exist");
+    assert!(
+        content.contains("7000"),
+        "persisted file should contain score 7000: {content}"
+    );
+    std::fs::remove_file(&save_path).ok();
+}
+
+#[test]
+fn check_high_score_does_not_update_when_score_equal() {
+    let mut world = bevy::ecs::world::World::default();
+    world.insert_resource(Score { value: 75 });
+    world.insert_resource(HighScore { value: 75 });
+    world.register_system(check_high_score);
+    world
+        .run_system_cached(check_high_score)
+        .expect("system should run");
+    let high_score = world.resource::<HighScore>();
+    assert_eq!(
+        high_score.value, 75,
+        "HighScore should remain unchanged when score equals high score"
+    );
+}
+
+#[test]
+fn check_high_score_does_not_update_when_score_lower() {
+    let mut world = bevy::ecs::world::World::default();
+    world.insert_resource(Score { value: 30 });
+    world.insert_resource(HighScore { value: 99 });
+    world.register_system(check_high_score);
+    world
+        .run_system_cached(check_high_score)
+        .expect("system should run");
+    let high_score = world.resource::<HighScore>();
+    assert_eq!(
+        high_score.value, 99,
+        "HighScore should remain unchanged when score is lower"
+    );
+}