Layout: minor optimizations and bugfix for non-functional details (#38197)

This change adds some minor optimizations and bugfix for non-functional
details with seperated commits:
- fix the omission that stop use `Rayon` in single-thread mode
- add trace for incremental box tree construction
- fix the bug that failed to skip reflow entirely when there is no need
for `restyle` and a fragment tree has already been built.
- add trace for stylist preparation during reflow. In certain scenarios,
this phase might take up a significant amount of time, such as when
there are a large number of shadow trees.

Testing: This should not change observable behavior and is thus covered
by existing WPT tests.

---------

Signed-off-by: sharpshooter_pt <ibluegalaxy_taoj@163.com>

components/layout/flow/root.rs

@@ -376,6 +376,7 @@ impl<'dom> IncrementalBoxTreeUpdate<'dom> {
         })
     }
 
+    #[servo_tracing::instrument(name = "Box Tree Update From Dirty Root", skip_all)]
     fn update_from_dirty_root(&self, context: &LayoutContext) {
         let contents = ReplacedContents::for_element(self.node, context)
             .map_or_else(|| NonReplacedContents::OfElement.into(), Contents::Replaced);