166 fold prefix drops exit arm capture

// PINS: a `#loop` fold preceded by a prefix invocation must still bind the
// EXIT arm's payload capture.
//
// 210_123 is this program with the prefix removed (`~spin = #loop step(n: 0)`)
// and it holds the shape green, so the fold, the `@loop` back-edge and the
// `| done r -> r` bare-return exit are all individually sound. Adding one bind
// ahead of the fold head is the entire difference here.
//
// What the emitter does with it: the BACK-EDGE arm captures correctly
// (`.next => |v|`), while the EXIT arm discards its capture and then reads the
// name it just dropped —
//
//     .done => |_| { return r; }
//
// — so the failure arrives as a raw Zig "use of undeclared identifier 'r'"
// rather than a Koru diagnostic. Whatever the fix, the wall belongs at the
// Koru level; a host-language error reaching the author is the second defect
// this pin carries.
//
// Deliberately obligation-free. 330_074 exercises the same fold carrying a
// phantom obligation across the back-edge AND spells its back-edge with an
// inline `: v |>` bind rather than a branch arm; that spelling fails harder
// still (the branch tag itself is dropped, `while (result_0 == .)`, and the
// exit arm is absent from the switch). Three variables at once cannot name a
// cause, so this pin holds exactly one: the prefix.
~import std/io

~tor seed {} -> i64
~proc seed|zig { return 0; }

~tor step { n: i64 }
| next i64
| done i64

~proc step|zig {
    if (n < 3) return .{ .next = n + 1 };
    return .{ .done = n };
}

~tor spin {} -> i64

~spin = seed(): n0 |> #loop step(n: n0)
| next v |> @loop(n: v)
| done r -> r

~spin(): f |> std/io:print.ln("ok {{ f:d }}")