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.

config.toml

@@ -1,5 +1,8 @@
 initial_chunk_radius = 15
 
+[display]
+vsync = "mailbox"
+
 [spawn_counts]
 dorfs = 5
 pigs = 5