Third stdlib module. Either<e, a> is the first 2-type-var data def,
and the eliminator `either : (e -> c) -> (a -> c) -> Either<e, a> -> c`
the first fn with three type vars on top of the data — the deepest
polymorphism shipped end-to-end so far.
Five combinators: from_right, is_left, is_right, map_right, either.
Demo exercises every one and runs to deterministic output. IR shows
six distinct monomorphisations including two `from_right` variants
(Left=Int vs Left=$u depending on call site) and `either__I_I_I`.
No new compiler bugs surfaced: 14a / 14h / 15b coverage was wide
enough to handle 2-type-var data plus 3-type-var fns out of the box.
Discovered (queued as 15e): `ail render` and `ail parse` are not
symmetric. Form-(A) printer in ailang_surface::print is the actual
inverse of `parse` (round-trip test covers it across all 25 fixtures
including std_either) but is not exposed via the CLI; `render` still
calls the older ailang_core::pretty::module printer.
Tests: 89/89 (was 88, +1 e2e for std_either_demo).
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
Brummel
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