Iter 15g — std_either_list: first 3-way cross-module stdlib fn

First stdlib fn that imports three other stdlib modules (std_list,
std_either, std_pair) and returns a compound polymorphic ADT tree
(Pair<List<e>, List<a>>). Stresses monomorphisation across nested
parameterised ADTs.

What shipped:
- examples/std_either_list.{ailx,ail.json}: 3 combinators —
  - lefts : forall e a. (List<Either<e, a>>) -> List<e>
  - rights : forall e a. (List<Either<e, a>>) -> List<a>
  - partition_eithers : forall e a. (List<Either<e, a>>)
                        -> Pair<List<e>, List<a>>
  lefts/rights use depth-2 nested Ctor patterns (Cons (Left l) t)
  — exercises 16a's desugar pass at a depth not reached by any
  prior fixture.
- examples/std_either_list_demo.{ailx,ail.json}: drives all three
  combinators on a five-element List<Either<Int, Int>>; expected
  output one per line: 2, 3, 2, 3.
- crates/ail/tests/e2e.rs::std_either_list_demo: e2e count 34 → 35.
- docs/JOURNAL.md: Iter 15g entry.

Compiler bug surfaced (queued as 15g-aux, not fixed):
unify_for_subst in ailang-codegen/src/lib.rs accepts $u wildcards
only on the arg side. Mixing inline (Either Left n) and (Either
Right n) in a list literal lands $u on the param side via the
outer List<a>'s binding and errors. Reduced repro: `length [Left
1, Right 10]`. Demo works around with monomorphic mkleft/mkright
helpers; workaround documented inline. Fix is a one-line symmetric
extension of the early-return.

Tests: 94/94. Stdlib: 5 modules, 27 combinators.

crates/ail/tests/e2e.rs

@@ -395,6 +395,28 @@ fn std_either_demo() {
     );
 }
 
+/// Iter 15g: `std_either_list` end-to-end. First stdlib fn set that
+/// imports three other stdlib modules (`std_list`, `std_either`,
+/// `std_pair`) and returns a compound polymorphic ADT tree
+/// (`partition_eithers : List<Either<e,a>> -> Pair<List<e>, List<a>>`).
+///
+/// Property protected: monomorphisation across `List × Either × Pair`
+/// in a single call chain — `std_either_list.partition_eithers` is
+/// instantiated at `(e=Int, a=Int)` and the result feeds
+/// `std_pair.fst` / `std_pair.snd` (each at `(a=List<Int>, b=List<Int>)`)
+/// into `std_list.length` (at `a=Int`). If cross-module ctor resolution
+/// or substitution-through-nested-Con regressed, one of the
+/// instantiations would either fail to emit or produce a wrong length.
+/// The four expected outputs (2, 3, 2, 3) also pin the order
+/// preservation of `lefts` and `rights` against the input
+/// `[Left 1, Right 10, Left 2, Right 20, Right 30]`.
+#[test]
+fn std_either_list_demo() {
+    let stdout = build_and_run("std_either_list_demo.ail.json");
+    let lines: Vec<&str> = stdout.lines().collect();
+    assert_eq!(lines, vec!["2", "3", "2", "3"]);
+}
+
 /// Guards `ail diff`: a modified body changes the hash of `sum`, while
 /// `main` stays unchanged. Expects exit code 1, `changed` contains exactly
 /// `sum`, `unchanged` contains `main`, `added`/`removed` empty.