Fix/css inherit shorthand resolution

[jafin]

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Description

Fix CSS inherit keyword not resolving on shorthand properties

The inherit keyword on shorthand properties like border, margin,
and padding was not correctly resolved to parent values. Three
independent bugs contributed:

1. DeclarationInfo: shorthand converters with null InitialValue were
   not wrapped with StandardValueConverter, so CSS-wide keywords like
   inherit were either ignored or partially mis-parsed by the
   property-specific converter. Fixed by always wrapping with
   StandardValueConverter first.

2. DeclarationInfoExtensions.Expand: CssInheritValue and CssUnsetValue
   were not handled when expanding shorthand properties to longhands,
   causing them to fall through to the aggregator Split which returned
   null. Added explicit handling for both value types.

3. DeclarationInfoExtensions.Collapse: CssInheritValue was not tracked
   for round-trip serialization of shorthand properties. Added inherit
   tracking alongside initial, unset, and child.

4. CssStyleDeclaration.ChangeDeclarations: when UpdateDeclarations
   found a child property with explicit inherit value, it removed it
   but did not allow the parent value to replace it because the skip
   flag was not reset. Fixed by setting skip to false when the old
   declaration is removed.

fixes #184

[FlorianRappl]

LGTM - thanks!