[menshikovs]

Summary

Fix an issue where CSS @import rules can fail to load in Firefox during CSSV table updates.

Problem

In Firefox 155, appending an import-bearing stylesheet to an inert HTMLDocument can prevent an identical import from loading later in a connected Shadow DOM.

This affects CSSV's in-place stylesheet update path.

Solution

• Detect @import at-rules before entering the detached-document style probe.
• Fall back to full rendering when imports are present.
• Preserve in-place updates for ordinary stylesheet changes.
• Handle escaped and case-insensitive import names.
• Ignore import-like text inside comments and strings.

Testing

• Chromium: 91/91 browser tests passed.
• Firefox: 90/91 browser tests passed.
• Unit tests: 129/129 passed.

The remaining Firefox failure concerns the existing :lang() behavior across Shadow DOM boundaries ([Mozilla Bug 1579075](https://bugzilla.mozilla.org/show_bug.cgi?id=1579075)) and is unrelated to this change.

No specification changes are required.