Add clarification comment

Author: MDLC01

src/lib.rs

@@ -269,6 +269,8 @@ mod test {
             .collect::<HashSet<_>>();
         assert!(
             are_all_variants_valid(SYM, |c| {
+                // All text presentations are exactly 2 codepoints long as of Unicode 16.0.
+                // See https://github.com/typst/codex/pull/114#discussion_r2266318559.
                 !(c.chars().count() == 1
                     && require_presentation_selector.contains(&c.chars().next().unwrap()))
             }),
@@ -285,6 +287,8 @@ mod test {
             .collect::<HashSet<_>>();
         assert!(
             are_all_variants_valid(EMOJI, |c| {
+                // All text presentations are exactly 2 codepoints long as of Unicode 16.0.
+                // See https://github.com/typst/codex/pull/114#discussion_r2266318559.
                 !(c.chars().count() == 1
                     && require_presentation_selector.contains(&c.chars().next().unwrap()))
             }),