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)]