typecheck: preserve lowercase return declarations

README.md

@@ -196,6 +196,8 @@ Function square declares an unknown return type BANANA. Use a Claro type such as
 
 The same check applies to methods, so `Player.score RETURNS BANANA` reports `Method Player.score declares an unknown return type BANANA...` instead of allowing a misspelled type into a class definition. Complete `IF`/`ELSE` return branches are also accepted inside typed functions and methods, including a nested conditional whose own branches all return; an incomplete path still gets a missing-return diagnostic.
 
+Claro keywords are case-insensitive here, so `returns NUMBER` is accepted as the same return declaration as `RETURNS NUMBER`. The type checker keeps the return-value diagnostic when a lowercase declaration is used, which helps learners who are still learning Claro's capitalization style.
+
 The unknown-function diagnostic is validated for both modern `DO squre 4` and compatibility `CALL squre WITH 4` calls. Compatibility calls that leave `WITH` empty, such as `CALL greet WITH`, now count as zero arguments, so learners get the same missing-argument guidance as `DO greet` instead of the checker treating the blank as an argument.
 
 For functions with more than one checked parameter, Claro reports each mismatched argument with the parameter name: