005 missing branch comp error

// Template engine: `{% comp error %}` — a template makes a branch REQUIRED by
// referencing it. The consumer omits `| done`, so `continuations["done"]` is
// absent and the template raises a fantastic, author-defined diagnostic
// instead of letting a cryptic host-language error through. The template body
// IS the contract: by checking for `done`, the template declares it required.
~import std/io

~pub event probe2 { n: usize }
! each usize

~proc probe2|template|zig {
    {% if continuations["done"] %}{% else %}{% comp error %}probe2 requires a `done` branch{% endcomp %}{% endif %}
    for (0..{{ n }}) |__i| {
        {% for h in effects["each"] %}
        {{ h.link }}(__i);
        {% endfor %}
    }
}

~probe2(n: 3)
! each i |> std/io:print.ln("each {{ i:d }}")