Clarify that attribution rectangles use CSS pixels

[anediaz]

Summary

This PR clarifies that previousRect and currentRect use CSS pixels, consistent with other Web Platform APIs like getBoundingClientRect(), IntersectionObserver, and ResizeObserver.

Changes:

• Added note after LayoutShiftAttribution interface explaining CSS pixel usage
• Updated "create the attribution" algorithm to explicitly specify CSS pixels

Implementation: https://chromium-review.googlesource.com/c/chromium/src/+/6624567
ChromeStatus: https://chromestatus.com/feature/5155103518228480

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Preview | Diff

[anediaz]

👋 Hi @skobes-chromium @npm1 @eyaich - requesting review of this spec clarification.

This PR documents that attribution rectangles use CSS pixels (not device pixels), aligning the spec with Chrome's implementation that shipped in early October.

Status:

• Chrome CL merged: https://chromium-review.googlesource.com/c/chromium/src/+/6624567
• Target: Chrome M144
• Mozilla: No concerns (approved)
• WebKit: Position requested
• Privacy & Security: Approved

cc @mmocny - would appreciate your review as the Layout Instability expert and the ultimate reviewer of this fix.

Happy to address any feedback!