fix(PR-6192): name keyword choice parser result

Use a private alias for the parser's three-state keyword-choice payload so the strict outcome remains clear without tripping Clippy type complexity.

Co-authored-by: Try everything <tryeverything24@users.noreply.github.com>

Author: matthewevans

crates/engine/src/parser/oracle_effect/subject.rs

@@ -3399,9 +3399,9 @@ fn build_keyword_choice_sub_ability(
     ))
 }
 
-fn parse_keyword_choice_grant(
-    predicate: &str,
-) -> Result<Option<(Vec<Keyword>, Option<Duration>)>, ()> {
+type KeywordChoiceGrant = (Vec<Keyword>, Option<Duration>);
+
+fn parse_keyword_choice_grant(predicate: &str) -> Result<Option<KeywordChoiceGrant>, ()> {
     let lower = predicate.to_lowercase();
 
     // CR 608.2d: "gain your choice of X, Y, or Z" offers a resolution-time choice.