Using Future::OP with _DefaultImpl or not?

question.txt

Hit this working on an async redis client - a pubsub message handler
disconnects two connections, which cancels both their in-flight reads,
and with the default impl that dies:

    Can't call method "fh" on an undefined value at Future/IO.pm line 890

Best I can tell the poll loop caches $npollers and then completes ready
futures inside the loop, so when a continuation cancels other pending
reads, on_cancel splices them out of @pollers behind the loop's back and
the index runs off the end.

Boiled it down to this. On the stock default impl it dies every time;
run the same script with -MFuture::IO::Impl::IOAsync and it completes
fine, so it looks specific to _DefaultImpl's poll loop:

    use v5.36;
    use Socket;
    use Future::IO;

    my (@rd, @wr);
    for (0..2) {
        socketpair my $r, my $w, AF_UNIX, SOCK_STREAM, PF_UNSPEC or die $!;
        push @rd, $r;
        push @wr, $w;
    }

    my @f = map { Future::IO->read($_, 10) } @rd;
    $f[0]->on_done(sub { $f[1]->cancel; $f[2]->cancel });
    syswrite $wr[0], "hello";
    $f[0]->get;
    say "completed without error";

I patched my local copy to collect the ready pollers during the scan and
fire ->done after it finishes (with an is_ready guard) and that fixes the
repro, plus my app's full test suite passes on it. Happy to send the
patch if you want.