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Iter 15f: std_pair — 2-type-var product, no recursion

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Fourth stdlib module. Pair<a, b> is the canonical product with two
type vars and a single constructor MkPair. Five combinators: fst,
snd, swap, map_first (Pair<a, b> -> Pair<c, b>), map_second
(Pair<a, b> -> Pair<a, c>).

Smallest dogfood for the parameterised-ADT path so far: no
recursion in the data def or any combinator, single-arm matches
throughout. Demo prints 7, 9, 9, 7, 8, 18 deterministically.

No compiler bug surfaced (fourth stdlib iter in a row to land
clean). The only fixable issue was a paren-balance typo in the
demo's seq chain.

Cumulative: 4 stdlib modules, 24 combinators. Type-system surface
exercised end-to-end now spans 1/2/3-type-var data, 1/2/3-type-var
fns, recursive ADTs, cross-module imports of all of the above,
flat and nested patterns, TCO via monomorphised musttail, GC.

Tests: 93/93 (e2e 33 → 34).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
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@@ -2768,3 +2768,50 @@ trees) plug in at the same desugar layer.
**Queue update.** 16a done. Remaining: 15f (`std_pair`, optional);
16b (local recursive `let`); 17a (per-fn arena); future
"lit-in-Ctor" follow-up under 16c if and when needed.
---
## Iter 15f — `std_pair`: 2-type-var product, no recursion
**Goal.** Round out the small-ADT stdlib triad (Maybe, Either, Pair).
Pair is the canonical product with two type vars and a single
constructor — no recursion in the data def or any combinator.
Smallest dogfood for the parameterised-ADT path so far.
**What shipped.**
- `examples/std_pair.ailx` (~75 LOC, 5 combinators):
- `fst`, `snd` — projections.
- `swap : Pair<a, b> -> Pair<b, a>`.
- `map_first : (a -> c) -> Pair<a, b> -> Pair<c, b>`.
- `map_second : (b -> c) -> Pair<a, b> -> Pair<a, c>`.
Each combinator is a single-arm match on `MkPair` with no
fall-through, so the desugar pass added in 16a is a no-op
(the patterns are already flat).
- `examples/std_pair_demo.ailx` exercises every combinator. Output
(deterministic): 7, 9, 9, 7, 8, 18.
- `crates/ail/tests/e2e.rs::std_pair_demo` guards the path.
**No new compiler bug surfaced.** Stdlib iter #4 in a row that
landed clean. The only fixable issue was a paren-balance typo in
the demo's `seq` chain, surfaced immediately by the parser's
"unexpected end of input, expected `)`" diagnostic.
**Tests: 93/93** (e2e went from 33 to 34).
**Cumulative state, post-15f.**
- Stdlib: 4 modules (`std_maybe`, `std_list`, `std_either`,
`std_pair`); 24 combinators total.
- Type-system surface area exercised end-to-end: 1- and 2- and
3-type-var data; 1- and 2- and 3-type-var fns; recursive ADTs;
cross-module imports of all of the above; flat *and* nested
patterns (16a); TCO via monomorphised `musttail`; Boehm GC.
- Pipeline layers: `load → desugar → check → codegen → clang`.
Each layer has a public, narrow contract; the desugar layer
is the natural home for further surface-level smoothing
without enlarging the core AST.
**Queue update.** 15f done. Remaining: 16b (local recursive `let`),
16c (Lit-in-Ctor patterns), 17a (per-fn arena). None blocking
further stdlib growth; each is a quality-of-life improvement.