When a shorthand like `background` containing `var()` is used in
a `::selection` rule, the shorthand was filtered out by the pseudo-
element property whitelist before variable resolution could occur.
This left PendingSubstitutionStyleValue longhands unresolved,
causing either a crash or incorrect computed values.
Allow unresolved shorthands to bypass the pseudo-element filter so
variable resolution can proceed. After resolution and expansion
into longhands, filter out any that the pseudo-element does not
support.
Fixes#8625.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
