Restore the missing details-marker reftest reference in the WPT profile #867

Parent: #227
Related CSS epic: #235

Failure

WebScene PR CI deterministically fails in the Build and test Linux X64 job while listing the WebPlatformSubset manifest:

System.IO.FileNotFoundException: Reference for 'contracts/css-webkit-details-marker-display.html' is missing.
  at WebScene.WebPlatformSubset.Runner.WptSubsetRunner.ValidateManifest()

The test and contracts/css-webkit-details-marker-display-ref.html are both tracked. The manifest entry is type: reftest but omits its required reference property, so ValidateManifest() fails before any test can run. This is independent of the PR under test and leaves ordinary WebScene CI structurally red.

Observed in WebScene PR #865, Actions run 35522483565, job 106108845869.

Fix

Add the existing reference path to the profile entry:

"reference": "contracts/css-webkit-details-marker-display-ref.html"

Keep the authored <link rel="match"> unchanged. Validate JSON parsing and the runner's full --selection all --list manifest check. No CSS runtime, layout, paint, visual-tree, or application behavior changes.

Acceptance

• WebPlatformSubset manifest validation lists all selections successfully.
• The details-marker reftest resolves its tracked reference.
• Normal WebScene Linux CI proceeds beyond manifest validation.