115 effect event via subflow

// SUBFLOW-IMPLEMENTED EFFECTS (Barbour effect-typing arc) — the historic pin,
// now on the RULED spelling (2026-07-02): firing an arm IS calling it, with
// established subflow grammar only. The original sketch's statement-fire
// (`gen => for(0..n) ! each i | done n`) is dead; inside the declaring
// event's impl, the event's own arms are CALLABLE, exactly as a `proc` body
// calls `each(i)` today — so the firing chains with stock `|>` and the
// terminal maps with stock `=>` (240's shape).
//
// Why it matters (unchanged from the original pin): every effect-bearing
// event used to be `proc`-backed — the proc was the trust boundary, firing
// effects from opaque host code "unsafe"-style. With a subflow impl, the
// firing and the per-firing obligation rule are visible to the flow-level
// checkers instead of trusted. Pure-.k twin: 400_131; obligation pair:
// 400_136 (balanced, runs) / 400_137 (escape, KORU030).
import std/io
import std/control

pub tor gen { n: usize }
! each usize
| done usize

gen = for(0..n)
! each i |> each(i)
| done => done n

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