fix(w3c/style): disable the dark stylesheet before inserting the link

[marcoscaceres]

Refs #5436.

Since v37.3.3 a spec that never opted into dark mode paints the wrong color scheme on roughly one cold load in five in Chrome, in both directions, because ReSpec assigned disabled to the dark stylesheet link on the statement after inserting it, and a write that lands mid-fetch leaves Chrome applying the sheet anyway and dropping it on the next write. This sets the attribute before insertion instead, so the link is never in the document both enabled and still loading.

AI generated the logic here. Gemini wrote the test from the two reports before the fix existed; only the light-system direction is pinned, because a dark-system spec passed on the base commit in 5 of 5 runs and so could never fail without the fix.

Proof: npx jasmine --random=false tests/darkmode-race.cjs fails on the base commit in 3 of 3 runs with Expected 'rgb(11, 22, 33)' to be 'rgb(255, 255, 255)', and passes in 3 of 3 on the head commit. That color is what the test serves in place of dark.css, so a failure means the dark stylesheet applied for a reader whose system asked for light.

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Pull request overview

Prevents Chrome from briefly enabling the W3C dark stylesheet before its disabled state is established.

Changes:

• Sets disabled before inserting the dark stylesheet.
• Adds integration and Puppeteer regression coverage.
• Includes the race test in headless testing.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 6 comments.

File	Description
src/w3c/style.js	Disables the dark stylesheet before insertion.
tests/spec/w3c/style-spec.js	Updates stylesheet-state coverage.
tests/darkmode-race.cjs	Adds a Chrome race regression test.
package.json	Runs the regression with headless tests.

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[marcoscaceres]

Confirmed working as expected locally. Ran 50 times in chrome, no race condition.