014 for template cross target

// FRONTIER: ~for now HONESTLY selects `for|template|js` on a --lang=js build
// (koru_std/control.kz), and the JS emitter resolves BOTH splice-marker kinds
// the template mints:
//   - the per-iteration `! each` effect splice (`__koru_inline_scoped_0(item)`)
//     lowers to `{ const _x = item; <handler body> }` — RESOLVED, no longer a
//     leaked, unresolved marker; and
//   - the `| done |> _` terminal's `__koru_continue_N` lowers to an empty block,
//     as `_` should.
// Proven with a JS-array iterable: `~for([1,2,3]) ! each _ |> println(...)`
// emits a runnable `for (const x of [1,2,3]) { … }` that prints tick×3.
//
// The ONE remaining gap is the iterable EXPRESSION: `{{ iterable }}` substitutes
// `0..3` verbatim, so the JS body becomes `for (const x of 0..3)` — `0..3` is
// Zig range syntax, not a JS iterable, and node rejects it. Closing this needs
// range→JS lowering (`0..N` → a counting loop or `Array.from`/generator), a
// target-aware transform of the iterable expression that does not exist yet.
// That is the whole of what's left; the for-body + effect-splice lowering is done.
~import std/control
~import std/io

~for(0..3)
! each _ |> std/io:print("tick\n")
| done |> _