Iter 15g-aux — symmetric $u early-return in unify_for_subst

Fix the codegen asymmetry that 15g surfaced. unify_for_subst had an
arg-side-only early-return for $u-prefixed synth wildcards. The
function's prev-binding recursion can swap a $u from arg into param
position when re-unifying a previously-bound type against a fresh
arg. Reduced repro: `length [Left 1, Right 10]` — `a` first binds
to `Either<Int, $u>`, then the recursive unification against
`Either<$u, Int>` lands $u in param-pos[1] and falls through to
the catch-all error.

Fix is three lines: add a symmetric $u early-return for param side.
$u is a synth-only wildcard regardless of which side it ends up on
after the prev-binding swap. Doc comment expanded to record the
origin and justification.

std_either_list_demo refactored: mkleft/mkright workaround helpers
removed; the list is now constructed inline by mixing
(term-ctor std_either.Either Left 1) and (... Right 10) directly.
Same expected output (2, 3, 2, 3); the demo doubles as the 15g-aux
regression fixture.

Tests: 94/94, unchanged. The fix expanded what compiles without
changing observable behaviour for any prior fixture.

docs/JOURNAL.md

@@ -2991,3 +2991,54 @@ under `ailang-codegen/src/lib.rs`.
 acceptance — small, one-line in `unify_for_subst`); 16b (local
 recursive `let`); 16c (Lit-in-Ctor patterns); 17a (per-fn arena,
 gated on user discussion).
+
+---
+
+## Iter 15g-aux — symmetric `$u` early-return in `unify_for_subst`
+
+**Goal.** Fix the codegen asymmetry that 15g surfaced: inline list
+literals mixing `Left n` and `Right n` ctor calls of the same
+`Either<e, a>` errored at synth time even though both elements have
+fully-determined concrete-after-pinning types.
+
+**Diagnosis.** `unify_for_subst` (`crates/ailang-codegen/src/lib.rs`)
+had an arg-side-only early-return for `$u`-prefixed synth wildcards.
+The function has a prev-binding recursion path that re-invokes
+itself with the previously-bound type as the new param and the
+fresh arg, which can swap a `$u` from arg position into param
+position. Concretely, `length [Left 1, Right 10]` synths the list
+elements as `Either<Int, $u>` and `Either<$u, Int>`. The first
+element pins `a = Either<Int, $u>` for `Cons`'s parameter `a`. The
+second element triggers a recursive unification of the previously-
+bound `Either<Int, $u>` against `Either<$u, Int>`, walking
+pairwise: `Int` vs `$u` (arg-side `$u`, ok) and `$u` vs `Int`
+(param-side `$u`, **falls through** to the catch-all error).
+
+**Fix.** Three lines in `unify_for_subst`: add a symmetric early-
+return for param-side `$u`. Doc comment expanded to record the
+asymmetry's origin and the symmetric extension's justification
+(`$u` is a synth-only wildcard regardless of which side it ends
+up on after the prev-binding swap).
+
+**Demo refactor.** `examples/std_either_list_demo.ailx` no longer
+uses the `mkleft`/`mkright` monomorphic helpers introduced as the
+15g workaround. The list is now constructed inline by mixing
+`(term-ctor std_either.Either Left 1)` and `(term-ctor
+std_either.Either Right 10)` directly. Same expected output
+(2, 3, 2, 3); the demo doubles as the 15g-aux regression fixture.
+
+**Tests: 94/94, unchanged.** The fix expanded what compiles, did
+not change observable behaviour for any prior fixture.
+
+**Cumulative state, post-15g-aux.**
+
+- Stdlib unchanged (5 modules, 27 combinators).
+- One latent codegen bug retired. The bug count in the dogfood
+  audit since 14a stays at 4 surfaced + fixed (this is a fresh
+  surface from 15g, so 5 surfaced / 5 fixed).
+- The std_either_list_demo workaround is gone, leaving the inline
+  cross-module mixed-ctor list as the canonical idiom.
+
+**Queue update.** 15g-aux done. Unchanged: 16b (local recursive
+let), 16c (Lit-in-Ctor patterns), 17a (per-fn arena, gated on
+user discussion of memory management).