feat(niri): enable on-demand VRR for games

Turn on adaptive sync at the output only when a game window opts in, keeping desktop compositing fixed-rate to avoid idle/low-load VRR flicker. The Steam/Proton game rule enables VRR alongside the fullscreen rule.

Assisted-by: DeepSeek Harness (deepseek-flash)

home/linux/gui/niri/conf/windowrules.kdl

@@ -90,6 +90,7 @@ window-rule {
     exclude title="^(公告|Announcement|Notice)$"
     open-on-workspace "4entertainment"
     open-fullscreen true
+    variable-refresh-rate true
 }
 window-rule {
     match app-id="heroic"