171 if no else under midchain fold

// PIN: an omitted `| else` inside a `for` `! each` body inside a MID-CHAIN
// `#loop` fold arm must render empty — it must not exit the fold.
//
// THE SHAPE: `seed(): s |> #loop step(n: s)` puts the fold in mid-chain
// position (the spot 210_167 rules legal and holds green for outer
// bindings), and the `| more` arm carries the 2111 nesting — `for(...)`
// `! each i |> if(i == 2) | then |> ...` with the optional `| ?else` arm
// omitted. Every piece is green alone: `| ?else` is optional by
// declaration and 330_047 pins `! each _ |> if(c) | then |> _`.
//
// THE INVARIANT: a `.terminal` step that ends a chain INSIDE a spliced
// sub-construct (here, the absent else's continue inside an each-handler
// body) means "end this nested chain", never "exit the fold". Only a
// `.terminal` at the fold arm's own top level may lower to `break :label`
// — that one is the designed "arm ends without @loop" exit. The
// flow-head `#L` driver was repaired for exactly this (43d9a380f — nested
// terminals emit nothing); this pin holds the sibling driver to the same
// rule. The indicted machinery is the `ctx.current_label` the mid-chain
// `label_with_invocation` path in emitter_helpers.zig keeps set across the
// whole arm switch: a nested `.terminal` reaching it lowers to
// `break :label`, so the absent else exits the loop on the first
// non-matching iteration instead of rendering empty.
//
// Expected: the else-less `if` skips silently for i=1, prints once per
// round for i=2, and the fold runs to its `| done` exit.
~import std/io
~import std/control

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

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

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

~seed(): s |> #loop step(n: s)
| more m |> for(1..3)
    ! each i |> if(i == 2)
        | then |> std/io:print.ln("hit {{ m:d }}:{{ i:d }}")
    | done |> @loop(n: m)
| done d |> std/io:print.ln("done {{ d:d }}")