015 reject branch payload unbound

// TEST: Reject `| ok |> _` when the branch carries a payload.
//
// A payload-bearing branch must explicitly bind or discard its payload —
// silently ignoring the carried value is illegal. The branch name is part
// of the dispatch, but the value the branch carries must be acknowledged.
//
// Legal:   | ok x |> use(x)     — bind and use
// Legal:   | ok _ |> _          — explicit discard
// Illegal: | ok |> _            — neither bound nor discarded

~tor compute {}
| ok i32
| err i32

~proc compute|zig {
    return .{ .ok = 42 };
}

~compute()
| ok |> _
| err _ |> _