ie-css: implement percentage resolution and calc() in style computation

[thomasnemer]

Goal

Percentage values and calc() expressions are parsed but not resolved.

Current state

• CssValue::Percentage(v) passes through to layout unchanged
• calc() is not parsed at all
• Layout receives percentage values but doesn't resolve them against containing block

Implementation needed

• Percentage resolution in resolve.rs: width % of containing block, font-size % of parent, etc.
• calc() parsing: basic arithmetic (add, sub, mul, div) with mixed units
• calc() resolution at compute time

Impact

Many CSS layouts use percentages (width: 50%, margin: 0 auto with %). calc() is increasingly common.