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.

examples/std_either_list.ailx

@@ -0,0 +1,71 @@
+; Iter 15g — fifth stdlib module: first 3-way cross-module fn set.
+; Imports std_list, std_either, std_pair simultaneously. All three
+; combinators dispatch over List<Either<e, a>>; partition_eithers
+; additionally returns Pair<List<e>, List<a>>. Stresses
+; monomorphisation across List × Either × Pair, cross-module ctor
+; resolution at qualified term-ctor sites, and the 16a desugar
+; layer at depth-2 nested Ctor patterns (in `lefts` / `rights`).
+
+(module std_either_list
+
+  (import std_list)
+  (import std_either)
+  (import std_pair)
+
+  (fn lefts
+    (doc "Project the Left payloads of a list of Eithers into a List<e>. Order-preserving. Uses the 16a nested-Ctor pattern to inline the inner Either dispatch within each Cons arm; the trailing wildcard arm seals the desugar's fall-through chain into a List<e> rather than the synthetic Unit fallback.")
+    (type
+      (forall (vars e a)
+        (fn-type
+          (params (con std_list.List (con std_either.Either e a)))
+          (ret (con std_list.List e)))))
+    (params xs)
+    (body
+      (match xs
+        (case (pat-ctor Cons (pat-ctor Left l) t)
+          (term-ctor std_list.List Cons l (app lefts t)))
+        (case (pat-ctor Cons (pat-ctor Right _) t)
+          (app lefts t))
+        (case _ (term-ctor std_list.List Nil)))))
+
+  (fn rights
+    (doc "Project the Right payloads of a list of Eithers into a List<a>. Symmetric to lefts; same nested-Ctor + wildcard-tail pattern shape.")
+    (type
+      (forall (vars e a)
+        (fn-type
+          (params (con std_list.List (con std_either.Either e a)))
+          (ret (con std_list.List a)))))
+    (params xs)
+    (body
+      (match xs
+        (case (pat-ctor Cons (pat-ctor Left _) t)
+          (app rights t))
+        (case (pat-ctor Cons (pat-ctor Right r) t)
+          (term-ctor std_list.List Cons r (app rights t)))
+        (case _ (term-ctor std_list.List Nil)))))
+
+  (fn partition_eithers
+    (doc "Partition a list of Eithers into a Pair<List<e>, List<a>>: lefts on the left, rights on the right. Single pass via direct recursion; head dispatch is a flat match on the recursive result's projections.")
+    (type
+      (forall (vars e a)
+        (fn-type
+          (params (con std_list.List (con std_either.Either e a)))
+          (ret (con std_pair.Pair (con std_list.List e) (con std_list.List a))))))
+    (params xs)
+    (body
+      (match xs
+        (case (pat-ctor Nil)
+          (term-ctor std_pair.Pair MkPair
+            (term-ctor std_list.List Nil)
+            (term-ctor std_list.List Nil)))
+        (case (pat-ctor Cons h t)
+          (let rest (app partition_eithers t)
+            (match h
+              (case (pat-ctor Left l)
+                (term-ctor std_pair.Pair MkPair
+                  (term-ctor std_list.List Cons l (app std_pair.fst rest))
+                  (app std_pair.snd rest)))
+              (case (pat-ctor Right r)
+                (term-ctor std_pair.Pair MkPair
+                  (app std_pair.fst rest)
+                  (term-ctor std_list.List Cons r (app std_pair.snd rest)))))))))))