076 effect branch nested in void chain

// Test: effect-bearing event invoked from inside a void chain.
//
// `setup()` is a plain void event. `emitter(...)` is a multi-fire pump
// that yields `! item usize` per element. The dispatch chains them:
//
//   ~setup() |> emitter(n: 3)
//   ! item i |> ...
//
// The outer flow's first invocation (`setup`) carries no effect branches,
// but the SECOND step in the void chain (`emitter`) does. The synthesized
// Handlers struct + comptime handler arg must be installed for the
// chained invocation, not the top-level one.
//
// Without the fix, `pending_handlers_name` is only set for the outer
// `setup()` call (which has no effect branches), so the chained
// `emitter(...)` call is emitted without its Handlers — producing a
// signature mismatch and a backend compile error.

~import std/io

~pub event setup { }

~proc setup|zig {
}

~pub event emitter { n: usize }
! item usize

~proc emitter|zig {
    var i: usize = 0;
    while (i < n) : (i += 1) {
        item(i);
    }
}

~setup() |> emitter(n: 3)
! item v |> std/io:print.blk {
    item {{ v:d }}
}