Iter 15h — std_list extension: take, drop

Adds the two canonical prefix-slicing combinators to std_list.
First fns in std_list to combine `if` + Int arithmetic + recursive
ADT pattern in one body.

  take : forall a. (Int, List<a>) -> List<a>
       — first n elements (or all of xs if shorter than n).
  drop : forall a. (Int, List<a>) -> List<a>
       — list with first n elements removed (Nil if n >= length).

Both use `(if (<= n 0) ...)` as the base-case guard since
literal sub-patterns inside Ctor patterns aren't yet supported
(queued as 16c — would let take/drop collapse the if into the
match).

examples/std_list_more_demo: builds [1, 2, 3, 4, 5] once and
exercises take/drop at n=0, n=mid, n=overflow on each. Expected
output 0, 3, 5, 5, 3, 0 (one per line).

Tests: 95/95 (e2e went from 35 to 36). std_list grew from 10 to
12 combinators; total stdlib 5 modules / 29 combinators. No new
compiler bug surfaced.

Pre-existing std_list defs are byte-identical (verified via diff
on .ailx and round-trip on .ail.json), so the five downstream
importers (std_list_demo, std_list_stress, std_either_list,
std_either_list_demo, list_map_poly) need no regeneration.

examples/std_list.ailx

@@ -161,4 +161,36 @@
         (case (pat-ctor Cons h t)
           (if (app p h)
             (term-ctor List Cons h (app filter p t))
-            (app filter p t)))))))
+            (app filter p t))))))
+
+  (fn take
+    (doc "First n elements of xs (or all of xs if it has fewer than n).")
+    (type
+      (forall (vars a)
+        (fn-type
+          (params (con Int) (con List a))
+          (ret (con List a)))))
+    (params n xs)
+    (body
+      (if (app <= n 0)
+        (term-ctor List Nil)
+        (match xs
+          (case (pat-ctor Nil) (term-ctor List Nil))
+          (case (pat-ctor Cons h t)
+            (term-ctor List Cons h (app take (app - n 1) t)))))))
+
+  (fn drop
+    (doc "All elements of xs after the first n (or Nil if xs has fewer than n elements).")
+    (type
+      (forall (vars a)
+        (fn-type
+          (params (con Int) (con List a))
+          (ret (con List a)))))
+    (params n xs)
+    (body
+      (if (app <= n 0)
+        xs
+        (match xs
+          (case (pat-ctor Nil) (term-ctor List Nil))
+          (case (pat-ctor Cons _ t)
+            (app drop (app - n 1) t)))))))