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>
