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/config.rs

@@ -8,6 +8,37 @@ use std::sync::OnceLock;
 pub struct GameConfig {
     pub initial_chunk_radius: i32,
     pub spawn_counts: SpawnCounts,
+    #[serde(default)]
+    pub display: DisplaySettings,
+}
+
+#[derive(Debug, Deserialize, Clone, Default)]
+pub struct DisplaySettings {
+    #[serde(default)]
+    pub vsync: VsyncMode,
+}
+
+#[derive(Debug, Clone, Copy, Default, PartialEq, Eq)]
+pub enum VsyncMode {
+    #[default]
+    Vsync,
+    Mailbox,
+    Uncapped,
+}
+
+impl<'de> Deserialize<'de> for VsyncMode {
+    fn deserialize<D>(deserializer: D) -> Result<Self, D::Error>
+    where
+        D: serde::Deserializer<'de>,
+    {
+        let s = String::deserialize(deserializer)?;
+        match s.as_str() {
+            "vsync" => Ok(VsyncMode::Vsync),
+            "mailbox" => Ok(VsyncMode::Mailbox),
+            "uncapped" => Ok(VsyncMode::Uncapped),
+            _ => Ok(VsyncMode::Vsync),
+        }
+    }
 }
 
 #[derive(Debug, Deserialize, Clone)]

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,