Function x_count 

pub fn x_count(
    game: &GameState,
    card_id: CardId,
    expr: &str,
    sa: &SpellAbility,
) -> i32

Evaluate an X-count expression on a card. Mirrors Java’s AbilityUtils.xCount(Card c, String s, CardTraitBase ctb).

Delegates to the SVar resolution system which already handles Count$, Number$, SVar$, and most sub-expressions.