feat: runtime-configurable VSync via config.toml

[display]
vsync = "mailbox"   # "vsync" | "mailbox" | "uncapped"

VsyncMode enum with custom Deserialize — accepts string values in toml.
apply_vsync_setting system reads GameConfig on every frame, updates
Window.present_mode immediately on change. No restart needed.

src/world/mod.rs

@@ -1,6 +1,6 @@
 use crate::{
     camera,
-    config::GameConfig,
+    config::{GameConfig, VsyncMode},
     world::generation::{
         generate_chunk_fauna, generate_chunk_foliage, generate_chunk_forrestry,
         generate_chunk_weathering_and_precipitation, apply_terrain_blobs, spawn_terrain_tasks,
@@ -18,6 +18,21 @@ pub use chunks::management::*;
 pub use textures::management::*;
 pub use tiles::{prefabs::*, rendering::*, visibility::*};
 
+pub fn apply_vsync_setting(
+    config: Res<GameConfig>,
+    mut windows: Query<&mut Window>,
+) {
+    if config.is_changed() {
+        if let Ok(mut window) = windows.single_mut() {
+            window.present_mode = match config.display.vsync {
+                VsyncMode::Vsync => bevy::window::PresentMode::AutoVsync,
+                VsyncMode::Mailbox => bevy::window::PresentMode::Mailbox,
+                VsyncMode::Uncapped => bevy::window::PresentMode::AutoNoVsync,
+            };
+        }
+    }
+}
+
 pub struct WorldPlugin;
 
 impl Plugin for WorldPlugin {
@@ -58,6 +73,7 @@ impl Plugin for WorldPlugin {
                         .after(crate::entities::item::item_tile_management_system),
                     tiles::track_benchmark,
                     tiles::render_bench_report_system,
+                    apply_vsync_setting,
                     handle_tile_occlusion_updates,
                     tiles::spawn_tilemap_chunks,
                     tiles::on_camera_z_changed,