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

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>

crates/ail/tests/e2e.rs

@@ -361,6 +361,20 @@ fn nested_ctor_pattern_first_two_sum() {
     assert_eq!(lines, vec!["30"]);
 }
 
+/// Iter 15f: `std_pair` end-to-end. Polymorphic product type with
+/// two type vars and a single constructor; six combinators including
+/// the 3-type-var `map_first` / `map_second` reshapers. Property
+/// protected: `Pair<a, b> -> Pair<c, b>` and `Pair<a, b> -> Pair<a, c>`
+/// monomorphise correctly when each is exercised at distinct
+/// `(a, b, c)` triples — the codegen must emit the right field types
+/// for the substituted MkPair output.
+#[test]
+fn std_pair_demo() {
+    let stdout = build_and_run("std_pair_demo.ail.json");
+    let lines: Vec<&str> = stdout.lines().collect();
+    assert_eq!(lines, vec!["7", "9", "9", "7", "8", "18"]);
+}
+
 /// Iter 15d: `std_either` end-to-end. First stdlib ADT with two type
 /// parameters (`Either<e, a>`); first combinator with three type vars
 /// (`either : (e -> c) -> (a -> c) -> Either<e, a> -> c`).