fix: root element not establishing stacking context (#35390) (#36174)

- [X] `./mach build -d` does not report any errors
- [X] `./mach test-tidy` does not report any errors
- [X] These changes fix #35390 
- [X] There are tests for these changes

[Successful WPT
run](https://github.com/reesmichael1/servo/actions/runs/14097679625)
(which includes the new test files)

(I didn't make the formatting changes intentionally--those came from
`mach format` following `mach test-tidy`.)

---------

Signed-off-by: Michael Rees <mrees@noeontheend.com>

tests/wpt/meta/MANIFEST.json

@@ -102180,6 +102180,19 @@
         {}
        ]
       ],
+      "root-element-creates-stacking-context.html": [
+       "be7b027ee791d8d83d6fcecae60654eb22ff960c",
+       [
+        null,
+        [
+         [
+          "/css/CSS2/stacking-context/root-element-creates-stacking-context-ref.html",
+          "=="
+         ]
+        ],
+        {}
+       ]
+      ],
       "zindex-affects-block-in-inline.html": [
        "5a5c9e3dcca6171910f85790e13dc754acda58e0",
        [
@@ -405642,6 +405655,10 @@
        "65138c9a66713e1ed8c2f649751deb1ceb22eac3",
        []
       ],
+      "root-element-creates-stacking-context-ref.html": [
+       "9bb42041498c4a5b2b86e6b282ec49e603a0880b",
+       []
+      ],
       "zindex-affects-block-in-inline-ref.html": [
        "c0fdb338d08198cfd058ccdabd7766a9e57375fd",
        []