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.

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