Iter 15d: std_either ships — 2-type-var ADT + 3-type-var eliminator

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>

crates/ail/tests/e2e.rs

@@ -305,6 +305,26 @@ fn std_list_stress_1000_element_folds() {
     );
 }
 
+/// 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`).
+///
+/// Property protected: monomorphisation correctly distinguishes per-
+/// call-site instantiations, including (a) two `from_right` instances
+/// differing only in the unused `e` (Int vs $u) and (b) the
+/// 3-type-var `either__I_I_I` instantiation. If wildcard substitution
+/// regressed, one of those would either fail to emit or collide with
+/// another instantiation.
+#[test]
+fn std_either_demo() {
+    let stdout = build_and_run("std_either_demo.ail.json");
+    let lines: Vec<&str> = stdout.lines().collect();
+    assert_eq!(
+        lines,
+        vec!["42", "99", "true", "true", "42", "6", "101"]
+    );
+}
+
 /// Guards `ail diff`: a modified body changes the hash of `sum`, while
 /// `main` stays unchanged. Expects exit code 1, `changed` contains exactly
 /// `sum`, `unchanged` contains `main`, `added`/`removed` empty.