Resolve CSS variable values at the points where properties are applied,
inherited, propagated, and used to seed transitions. This prevents raw
var() values from leaking into widget state or transition start/end
values when style attributes, inherited properties, or state-dependent
rules are involved.
Also refresh style definitions when widgets load so selector-dependent
rules are evaluated with the final widget context, and add regression
coverage for inline style vars, inherited vars, parent-state child rules,
tab close opacity, and foreground tint transitions.
