fix: top-level RETURN and READFILE designator targets error cleanly

- RETURN outside any FUNCTION/PROCEDURE used to emit an i64/void ret
  inside i32 main — a verifier dump instead of a diagnostic. It is now a
  clean compile error (no signature means top level: pseudocode cannot
  name a function 'main', it is a reserved runtime symbol).
- READFILE "f.txt", r.f mis-parsed into an error cascade the same way
  INPUT r.f did before its fix; the target now consumes the designator
  and reports one clear 'must be a plain STRING variable' error.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

0 files changed, 0 insertions, 0 deletions