Add CSS `aspect-ratio` support

[FlorianRappl]

Description
aspect-ratio is not currently respected in sizing/layout calculations.

Scope

• Support authored aspect-ratio for block/replaced elements where applicable.
• Integrate with width/height auto resolution and intrinsic size constraints.
• Ensure interactions with min/max constraints are handled predictably.

Acceptance criteria

• Elements with aspect-ratio resolve computed box size correctly in common layout scenarios.
• Works with explicit width or height and with auto dimensions.
• Regression tests cover replaced and non-replaced cases.