C variables

# C's variables and pointers holding a C enum (owner, 2026-09-28: b). A C variable holds C's
# plain number, since reading a variable never fails; convert it with `to.Mode[…]`, and back
# with `to.int32[Mode.vi]`, which never fails. `ptr.read` of a C enum is a call, so it takes
# `otherwise` and fails `out_of_range` for a number no case has (7 here).
layout.C enum Mode { emacs = 0, vi = 1 }
extern.C fn malloc ['bytes': uint64] -> ptr.mut[Mode];
extern.C mut 'edit_mode': int32;
fn mode_at ['p': ptr[Mode]] -> int32 {
    no_check let 'm' := ptr.read['p'] otherwise Mode.emacs;
    match 'm' {
        emacs { return 10; }
        vi { return 11; }
    }
}
MAIN {
    no_check change 'edit_mode' = to.int32[Mode.vi];
    no_check let 'raw' := 'edit_mode';
    let 'current' := to.Mode['raw'] otherwise Mode.emacs;
    no_check let 'mem' := malloc[8];
    no_check ptr.write['mem', value 1];
    no_check ptr.write[ptr.at['mem', by 1], value 7];
    no_check let 'a' := mode_at['mem'];
    no_check let 'b' := mode_at[ptr.at['mem', by 1]];
    std::print.useful[to.int32['current'], " ", 'a', " ", 'b', newln];
}