Brummel
df5b76bd65
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>