205 for join after each dropped

// JOIN after `for`'s effect arms: an anonymous `|>` at the loop step's level is
// the construct's join (resolveAnonymousArms: "keyword events rejoin") and must
// emit after the loop — not be dropped with the absent `| done`. The chain's
// drop(r) rides the same tail: swallowing it leaks the obligation silently.
//
~import std/io
~import std/control

const std = @import("std");
const Res = struct { v: i64 };

~tor mark { tag: string }
~proc mark|zig { std.debug.print("MARK {s}\n", .{tag}); }
~tor grab {} -> *Res<res!>
~proc grab|zig {
    const r = std.heap.page_allocator.create(Res) catch unreachable;
    r.* = .{ .v = 7 };
    return r;
}
~tor drop { r: *Res<!res> }
~proc drop|zig {
    std.debug.print("DROPPED {d}\n", .{r.v});
    std.heap.page_allocator.destroy(r);
}
~tor use { r: *Res<res> }
~proc use|zig { std.debug.print("USE {d}\n", .{r.v}); }
~tor doit {} | ok | fail
~proc doit|zig { return .{ .ok = .{} }; }
~tor probe {} | hit | ?skip
~proc probe|zig { return .{ .hit = .{} }; }
~tor probe2 {} | hit | ?skip
~proc probe2|zig { return .{ .skip = .{} }; }
~grab(): r |> doit()
| ok     |> mark(tag: "pre")
        |> for(0..2)
            ! each _ |> mark(tag: "lap")
    |> mark(tag: "join")
    |> drop(r)
| fail |> _