refactor(ksuid): replace BinExpr with top-level expr helpers

Adds StringKSUIDExpr and BinaryKSUIDExpr for use with ExprValues, plus
Expr() methods on both types. Removes ExprStringKSUIDs and BinExpr.

Co-authored-by: Cursor <cursoragent@cursor.com>

ksuid_string.go

@@ -48,6 +48,11 @@ func (s StringKSUID) AsBinaryKSUID() BinaryKSUID {
 	return BinaryKSUID{KSUID: s.KSUID}
 }
 
+// Expr returns a Jet expression for this StringKSUID. See [StringKSUIDExpr].
+func (s StringKSUID) Expr() Expression {
+	return StringKSUIDExpr(s)
+}
+
 // Scan implements sql.Scanner for string-backed KSUID columns.
 func (s *StringKSUID) Scan(src any) error {
 	switch v := src.(type) {