ocaml: phase 6 List.sort upgraded to mergesort (+3 tests, 528 total)

The previous List.sort was O(n^2) insertion sort. Replaced with a
straightforward mergesort:

  split lst    -> alternating-take into ([odd], [even])
  merge xs ys  -> classic two-finger merge under cmp
  sort cmp xs  -> base cases [], [x]; otherwise split + recursive
                  sort on each half + merge

Tuple destructuring on the split result is expressed via nested
match — let-tuple-destructuring would be cleaner but works today.

This benefits sort_uniq (which calls sort first), Set.Make.add via
sort etc., and any user program using List.sort. Stable_sort is
already aliased to sort.

lib/ocaml/runtime.sx

@@ -146,14 +146,30 @@
 
      let rec sort cmp xs =
        begin
-         let rec insert x ys =
-           match ys with
-           | [] -> [x]
-           | h :: t -> if cmp x h <= 0 then x :: ys else h :: insert x t
+         let rec split lst =
+           match lst with
+           | [] -> ([], [])
+           | [x] -> ([x], [])
+           | x :: y :: rest ->
+             (match split rest with
+              | (a, b) -> (x :: a, y :: b))
+         in
+         let rec merge xs ys =
+           match xs with
+           | [] -> ys
+           | x :: xs' ->
+             (match ys with
+              | [] -> xs
+              | y :: ys' ->
+                if cmp x y <= 0 then x :: merge xs' (y :: ys')
+                else y :: merge (x :: xs') ys')
          in
          match xs with
          | [] -> []
-         | h :: t -> insert h (sort cmp t)
+         | [x] -> [x]
+         | _ ->
+           (match split xs with
+            | (a, b) -> merge (sort cmp a) (sort cmp b))
        end
 
      let stable_sort = sort

lib/ocaml/test.sh

@@ -1310,6 +1310,14 @@ cat > "$TMPFILE" << 'EPOCHS'
 (epoch 5054)
 (eval "(ocaml-run \"max_int + min_int\")")
 
+;; ── List.sort mergesort ──────────────────────────────────────
+(epoch 5060)
+(eval "(ocaml-run \"List.sort compare [5;2;8;1;9;3;7;4;6]\")")
+(epoch 5061)
+(eval "(ocaml-run \"List.sort (fun a b -> b - a) [3;1;4;1;5]\")")
+(epoch 5062)
+(eval "(ocaml-run \"List.length (List.sort compare [9;8;7;6;5;4;3;2;1;0])\")")
+
 EPOCHS
 
 OUTPUT=$(timeout 360 "$SX_SERVER" < "$TMPFILE" 2>/dev/null)
@@ -2081,6 +2089,11 @@ check 5052 "Int.rem 17 5"                 '2'
 check 5053 "Int.compare 5 3"              '1'
 check 5054 "max_int + min_int (host int)" '0'
 
+# ── List.sort mergesort ─────────────────────────────────────────
+check 5060 "sort 9-element list"          '(1 2 3 4 5 6 7 8 9)'
+check 5061 "sort with reverse cmp"        '(5 4 3 1 1)'
+check 5062 "sort 10 reversed -> length"   '10'
+
 TOTAL=$((PASS + FAIL))
 if [ $FAIL -eq 0 ]; then
   echo "ok $PASS/$TOTAL OCaml-on-SX tests passed"

plans/ocaml-on-sx.md

@@ -407,6 +407,15 @@ _Newest first._
   binary search tree (`type 'a tree = Leaf | Node of 'a * 'a tree *
   'a tree`) with insert + in-order traversal. Tests parametric ADT,
   recursive match, List.append, List.fold_left.
+- 2026-05-09 Phase 6 — List.sort upgraded from O(n²) insertion sort
+  to O(n log n) mergesort (+3 tests, 528 total). split + merge are
+  inner functions of sort; tuple destructuring on the split result is
+  expressed via nested match (pattern parser needs explicit
+  paren-wrapping of tuple patterns inside match arms in some places —
+  inline let-tuple destructuring on a match RHS would be cleaner if
+  multi-binding `let (a, b) = ...` were promoted, but this works
+  today). Should make sort-using baselines noticeably faster on
+  larger lists; existing sort_uniq automatically benefits.
 - 2026-05-09 Phase 4 — integer `/` is now truncate-toward-zero on
   ints, IEEE on floats. Both operands integral → host floor/ceil based
   on sign; otherwise host `/`. Fixes `Int.rem` (which was returning 0