Add support for the CSS aspect-ratio property

[maitredede]

Implements the CSS aspect-ratio property. Closes #46.

What

Parses aspect-ratio: auto || <ratio> (with <ratio> = <number [0,∞]> [ / <number [0,∞]> ]?) and applies the preferred aspect ratio during layout, following CSS Sizing Level 4 §4.

• Parsing/storage — new AspectRatio longhand, a consumeAspectRatio consumer, and a BoxStyle::aspectRatio() accessor returning { isAuto, value }. The value is stored in the generic property map (not inherited), like width/height.
• Replaced boxes — the CSS ratio is blended into ReplacedBox::computeAspectRatioInformation: a specified ratio overrides the natural one, unless auto was given and the element has a natural ratio of its own.
• Non-replaced block boxes — the automatic axis is derived from the definite one in BlockBox::computeWidth/computeHeight, reusing the existing constrain* and box-sizing (adjust*) helpers.

Box-sizing

Per the spec, the reference box depends on the value:

• intrinsic ratio and auto && <ratio> → content box;
• a bare <ratio> → the box designated by box-sizing (border box when box-sizing: border-box).

A degenerate ratio (a 0 side) behaves as auto.

Not covered (yet)

• Full "transferred size constraints" from CSS Sizing 4 (min/max transferred through the ratio) — handled best-effort via the existing constrain* functions.
• Dedicated integration for flex/grid/table items — the value is parsed and available via the accessor, but not consumed by those layouts.

Testing

Built with meson/ninja (no new warnings) and verified end-to-end by rendering to PNG and measuring the resulting box dimensions:

Case	Expected	Result
width:200px; aspect-ratio:2/1	200×100	✅
height:100px; width:auto; aspect-ratio:3/1	300×100	✅
width:150px; aspect-ratio:1/1; box-sizing:border-box; border:10px	border box 150×150	✅
width:100px; aspect-ratio:16/9	100×56.25	✅
<img> (40×20) no aspect-ratio	40×20 (unchanged)	✅
<img> width:80px; aspect-ratio:1/1	80×80 (CSS overrides intrinsic)	✅
<img> aspect-ratio:auto 1/1	40×20 (intrinsic wins)	✅