114 effect multi arm output

// PROPOSED FORM (Barbour effect-typing arc) — NOT yet supported; pinned failing
// to capture the baseline (Phase 1).
//
// An effect branch's OUTPUT is a TREE of named continuation arms, generalizing
// the single anonymous `-> T` resume (cf. 400_085 `! double i32 -> i32`). Here
// `! ask` yields an i32 and the handler resumes by SELECTING one arm —
// `| halved i32` or `| timeout`. This is "effects having effects → sessions":
// the effect's output is itself event-shaped. You never nest `!`; the
// composition lives in the `|` tree.
//
// The resume-by-named-arm spelling (`=> halved v`) reuses the existing
// branch-ctor resume grammar (cf. the subflow resume `~greet => greeting "..."`
// in 020_014), now in effect-resume position.
//
// FRONTIER (undesigned): how the `~proc` CONSUMES a multi-arm resume value
// (today `const r = ask(payload)` binds a single value; a sum needs handling).
// This pin documents the SHAPE; the proc-side mechanics are the design work.
~import std/io

~pub event request { payload: i32 }
! ask i32
    | halved i32
    | timeout
| done i32

~proc request|zig {
    const r = ask(payload);
    return .{ .done = r };
}

~request(payload: 20)
! ask n => halved n / 2
| done r |> std/io:print.ln("{{ r:d }}")