Iter 15b: std_list ships, three more compiler gaps closed

Second stdlib module. Tester wrote std_list.ailx (10 combinators,
164 LOC) and a consumer demo. std_list typechecked standalone;
demo did not, surfacing three compiler bugs:

1. Check-side: Iter 14h's qualify_local_types was applied to
   Term::Var cross-module lookup but not to ctor-field types in
   Term::Ctor synth or Pattern::Ctor resolution. First recursive
   cross-module ADT (List has Cons a (List a) — recursive Con
   self-ref) triggers the bug. std_maybe slipped through because
   Maybe's ctors have no recursive Con field.
2. Codegen-side: same gap mirrored across 4 sites in codegen
   (Term::Ctor synth, lower_ctor, lower_match) plus a tweak to
   unify_for_subst (recurse on re-bind instead of strict equality
   so sibling-derived List<Int> accepts nullary-ctor's List<$u>
   wildcard).
3. Const codegen: emit_const rejected non-literal const bodies.
   The demo's xs : List<Int> = Cons 1 (...) requires it. Fix:
   per-module const table, Term::Var resolution loads literal
   consts from global, inlines non-literal bodies. Bare and
   qualified refs both supported.

All three fixes carry an "Iter 15b" code comment at their site.
~349/25 LOC across ailang-check, ailang-codegen, e2e.rs.

Tests 85 -> 87. New e2e std_list_demo asserts 11-line stdout:
length 5, is_empty false/true, head via from_maybe, tail length,
append length, reverse head, map double head, filter is_even
length, fold_left sum, fold_right sum. New ailang-check unit
test cross_module_recursive_adt_term_and_pat_ctor covers both
the original bug and the symmetric pat-ctor latent twin.

Hash invariance: all pre-15b fixtures + std_maybe defs
bit-identical. 14a / 14e / 14h regressions all green.

Cumulative state: 2 stdlib modules (std_maybe, std_list), 14
combinators, cross-module recursive ADT working end-to-end.
Three compiler bugs surfaced + fixed in dogfood since 14a (each
dogfood iter has surfaced ≥1).

Authoring observation: form (A) at 10 combinators is fine; main
friction is paren-counting in nested seq chains, not the form
itself. n-ary seq would help but is sugar.

Plan 15c: 1000-element list stress test for fold_left (tail-call-
marked) vs fold_right (constructor-blocked).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
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; Iter 15b — second stdlib module: polymorphic singly-linked lists.
; Imports std_maybe so head/tail can return Maybe<a> / Maybe<List<a>>.
; All cross-module references use the qualified form per the Iter 14h
; convention: `std_maybe.Maybe` at type-name slots, `std_maybe.from_maybe`
; for fns. Bare ctor names (`Just`, `Nothing`) stay unqualified — once
; the type is resolved the ctor lookup is unambiguous.
(module std_list
(import std_maybe)
(data List (vars a)
(doc "Polymorphic singly-linked list: Nil or Cons<a, List<a>>.")
(ctor Nil)
(ctor Cons a (con List a)))
(fn fold_left
(doc "Tail-recursive left fold. The recursive call is in tail position and is marked.")
(type
(forall (vars a b)
(fn-type
(params (fn-type (params b a) (ret b))
b
(con List a))
(ret b))))
(params f acc xs)
(body
(match xs
(case (pat-ctor Nil) acc)
(case (pat-ctor Cons h t)
(tail-app fold_left f (app f acc h) t)))))
(fn fold_right
(doc "Right fold. Constructor-blocked: the recursive call is the second arg of f, NOT a tail position.")
(type
(forall (vars a b)
(fn-type
(params (fn-type (params a b) (ret b))
b
(con List a))
(ret b))))
(params f acc xs)
(body
(match xs
(case (pat-ctor Nil) acc)
(case (pat-ctor Cons h t)
(app f h (app fold_right f acc t))))))
(fn length
(doc "Length via fold_left. The accumulating lambda ignores the element and increments the counter.")
(type
(forall (vars a)
(fn-type
(params (con List a))
(ret (con Int)))))
(params xs)
(body
(app fold_left
(lam (params (typed c (con Int)) (typed _ a))
(ret (con Int))
(body (app + c 1)))
0
xs)))
(fn reverse
(doc "Reverse via fold_left with a flipping accumulator. Tail-recursive by virtue of the fold_left call.")
(type
(forall (vars a)
(fn-type
(params (con List a))
(ret (con List a)))))
(params xs)
(body
(app fold_left
(lam (params (typed acc (con List a)) (typed h a))
(ret (con List a))
(body (term-ctor List Cons h acc)))
(term-ctor List Nil)
xs)))
(fn is_empty
(doc "True iff the list is Nil.")
(type
(forall (vars a)
(fn-type
(params (con List a))
(ret (con Bool)))))
(params xs)
(body
(match xs
(case (pat-ctor Nil) true)
(case (pat-ctor Cons _ _) false))))
(fn head
(doc "Returns Just<a> of the first element, or Nothing for an empty list. Cross-module Maybe.")
(type
(forall (vars a)
(fn-type
(params (con List a))
(ret (con std_maybe.Maybe a)))))
(params xs)
(body
(match xs
(case (pat-ctor Nil) (term-ctor std_maybe.Maybe Nothing))
(case (pat-ctor Cons h _) (term-ctor std_maybe.Maybe Just h)))))
(fn tail
(doc "Returns Just<List<a>> of the tail, or Nothing for an empty list.")
(type
(forall (vars a)
(fn-type
(params (con List a))
(ret (con std_maybe.Maybe (con List a))))))
(params xs)
(body
(match xs
(case (pat-ctor Nil) (term-ctor std_maybe.Maybe Nothing))
(case (pat-ctor Cons _ t) (term-ctor std_maybe.Maybe Just t)))))
(fn append
(doc "Concatenate two lists. Constructor-blocked recursion: the recursive call is inside Cons, NOT a tail.")
(type
(forall (vars a)
(fn-type
(params (con List a) (con List a))
(ret (con List a)))))
(params xs ys)
(body
(match xs
(case (pat-ctor Nil) ys)
(case (pat-ctor Cons h t)
(term-ctor List Cons h (app append t ys))))))
(fn map
(doc "Polymorphic map. Constructor-blocked recursion.")
(type
(forall (vars a b)
(fn-type
(params (fn-type (params a) (ret b))
(con List a))
(ret (con List b)))))
(params f xs)
(body
(match xs
(case (pat-ctor Nil) (term-ctor List Nil))
(case (pat-ctor Cons h t)
(term-ctor List Cons (app f h) (app map f t))))))
(fn filter
(doc "Keep elements where p is true. Constructor-blocked when p holds; non-tail recursive call when p is false.")
(type
(forall (vars a)
(fn-type
(params (fn-type (params a) (ret (con Bool)))
(con List a))
(ret (con List a)))))
(params p xs)
(body
(match xs
(case (pat-ctor Nil) (term-ctor List Nil))
(case (pat-ctor Cons h t)
(if (app p h)
(term-ctor List Cons h (app filter p t))
(app filter p t)))))))
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; Iter 15b — second consumer demo.
; Imports both std_maybe and std_list. Exercises every combinator at
; least once. xs is a top-level fn `() -> List<Int>` (consts cannot
; reference user fns; the brief flagged this).
(module std_list_demo
(import std_maybe)
(import std_list)
(fn inc
(doc "Add 1 to an Int. Used as the (a -> b) arg to map.")
(type (fn-type (params (con Int)) (ret (con Int))))
(params x)
(body (app + x 1)))
(fn add
(doc "Binary +. Used as the fold accumulator op.")
(type (fn-type (params (con Int) (con Int)) (ret (con Int))))
(params a b)
(body (app + a b)))
(fn is_even
(doc "Predicate: x mod 2 == 0.")
(type (fn-type (params (con Int)) (ret (con Bool))))
(params x)
(body (app == (app % x 2) 0)))
(fn double
(doc "Multiply by 2. Used as the map arg.")
(type (fn-type (params (con Int)) (ret (con Int))))
(params x)
(body (app * x 2)))
(const xs
(doc "The canonical list [1,2,3,4,5] for the demo. Pure ctor expression, so a const works.")
(type (con std_list.List (con Int)))
(body
(term-ctor std_list.List Cons 1
(term-ctor std_list.List Cons 2
(term-ctor std_list.List Cons 3
(term-ctor std_list.List Cons 4
(term-ctor std_list.List Cons 5
(term-ctor std_list.List Nil))))))))
(fn main
(doc "Drive each std_list combinator once. Expected outputs (per line): 5, false, true, 1, 4, 10, 5, 2, 2, 15, 15.")
(type (fn-type (params) (ret (con Unit)) (effects IO)))
(params)
(body
(seq (do io/print_int (app std_list.length xs))
(seq (do io/print_bool (app std_list.is_empty xs))
(seq (do io/print_bool (app std_list.is_empty (term-ctor std_list.List Nil)))
(seq (do io/print_int (app std_maybe.from_maybe -1 (app std_list.head xs)))
(seq (do io/print_int (app std_list.length (app std_maybe.from_maybe xs (app std_list.tail xs))))
(seq (do io/print_int (app std_list.length (app std_list.append xs xs)))
(seq (do io/print_int (app std_maybe.from_maybe -1 (app std_list.head (app std_list.reverse xs))))
(seq (do io/print_int (app std_maybe.from_maybe -1 (app std_list.head (app std_list.map double xs))))
(seq (do io/print_int (app std_list.length (app std_list.filter is_even xs)))
(seq (do io/print_int (app std_list.fold_left add 0 xs))
(do io/print_int (app std_list.fold_right add 0 xs)))))))))))))))