152 pointfree choke catches failing stage

// Pins the CHOKE's RUNTIME catch (210_151 only proves it structurally — no stage
// there ever fails). A point-free chain where stage-a can produce `| failed`;
// the dedented `| failed` choke claims it across the chain; a failing call is
// caught. stage-a(-5) fails → desugar's replicated choke routes it → run
// produces `failed` → call site prints "caught: negative".
~import std/io

~pub tor stage-a { ctx: i64 }
| ctx i64
| failed string
~proc stage-a|zig {
    if (ctx < 0) return .{ .failed = "negative" };
    return .{ .ctx = ctx + 1 };
}

~pub tor stage-b { ctx: i64 }
| ctx i64
| failed string
~proc stage-b|zig { return .{ .ctx = ctx * 2 }; }

~pub tor run { ctx: i64 }
| ctx i64
| failed string
~run = stage-a
  |> stage-b
| failed f => failed f

~run(ctx: -5)
| ctx c |> std/io:print.ln("ok {{ c:d }}")
| failed f |> std/io:print.ln("caught: {{ f:s }}")