quest: plan a boxed iterator's folds as std's box reads them

quest/m3/README.md

@@ -21,6 +21,7 @@ generic std gap is planned here even when moq never calls it.
 - [A trait object in a struct's last field](/quest/m3/ts-dyn-tail.md) - `Rc<RefCell<dyn Trait>>` and the like translate, the last field as the object
 - [bytes' Chain as a Buf object](/quest/m3/ts-buf-chain.md) - `Chain::copy_to_bytes` across both halves translates, and with it a `Chain` as a `dyn Buf`
 - [A Buf object's size](/quest/m3/ts-dyn-buf-size.md) - a `dyn Buf` object holds only the methods some call dispatches to
+- [A boxed iterator folds as std's box does](/quest/m3/ts-box-folds.md) - a `Box<I>` reads by `next` where std's `Iterator` for `Box<I>` forwards nothing else
 - [by_ref of a mutable reference to an iterator](/quest/m3/ts-by-ref-place.md) - `Iterator::by_ref` of a `&mut I` in a local translates
 - [mem::take as a function value](/quest/m3/ts-take-value.md) - `mem::take`, `replace`, and `swap` passed as a function value translate
 - [Debug of a RefCell](/quest/m3/ts-refcell-debug.md) - `{:?}` of a `RefCell`, `Ref`, and `RefMut` translates as std's writes them

quest/m3/ts-box-folds.md

@@ -0,0 +1,24 @@
+# [S] A boxed iterator folds as std's box does
+
+## Goal
+
+[rust2-ts](/ts/README.md) reads a `Box<I>` of a sized iterator, owned or behind a `&mut`,
+as std's `Iterator` for `Box<I>` does: by `I`'s own `next`, `size_hint`, `nth`, `last`,
+`next_back`, `nth_back`, `len`, and `is_empty`, and by `next` or `next_back` for every
+other consumer, proven by the lang fixture's differential tests against native Rust.
+
+## Plan
+
+Found on 2026-11-03 probing what [#437](https://github.com/moq-dev/rust2/pull/437) left:
+the translation erases the box and folds what it holds by its own `fold`, where std's
+defaults read the box by `next`. A boxed `Chain` of mapped `vec::IntoIter`s matches native
+across `count`, `fold`, `for_each`, `find`, `position`, `max_by_key`, `rev().fold`, and
+`skip(1).count()`, since `Chain`'s folds read and drop in `next`'s order. A box of a `skip`
+of a zip std reads by index does not: `Box::new(a.map(f).zip(b.map(g)).skip(1)).count()`
+makes and drops the skipped pair whole, as `Skip`'s fold does, where native skips it a
+side at a time through the zip's `nth`.
+
+`rt.iterDyn` already reads a `dyn Iterator` this way, so a box of a sized iterator can be
+one too, with `last` forwarded to the iterator's own, as std's `BoxIter` specializes it.
+
+Public API: the translator's output.