[adeerkhan]

Two spellings of IO are easy to swap, and each one reported an error that
never named the other:

import Base

def main() -> IO<Unit>:
  do IO<Unit>:
    IO.print("hello")

Error:
- message  : a declared datatype (unknown: IO)
Location: main
3>| def main() -> IO<Unit>:
  |               ^^^^^^^

The reverse, do IO(Unit):, reported only expected '<' / observed '('.

IO is a base def, not an ADT, so a type written IO<...> fell through to the
"unknown datatype" message; and the do parser expected < without saying
what it was for. Both now name the pair and the position:

• a return type: a type: IO<...>is the do block's effect syntax; a return type is writtenIO(...)``
• a do block: an effect's arguments in <...>(a do block isdo IO<...>:, and the type is written IO(...))

The two tests pin both messages and exit 1:

• tests/check/io_effect_in_return_type.bend
• tests/parse/do_effect_type_spelling.bend

What I ran: the source compiler (bun bend2/main.ts) on both tests (output
matches the pinned #| lines), gates/repo.ts --gate (PASS 54/54), and a valid
IO(Unit) / do IO<Unit> program (prints hi). gates/test.ts needs the
cluster and BEND_HOC is not set here, so I did not run the full test gate; CI
will.