ocaml: phase 5.1 anagram_groups.ml baseline (group by canonical anagram, 3 groups)

canonical builds a sorted-by-frequency string representation:

  let canonical s =
    let chars = Array.make 26 0 in
    for i = 0 to String.length s - 1 do
      let k = Char.code s.[i] - Char.code 'a' in
      if k >= 0 && k < 26 then chars.(k) <- chars.(k) + 1
    done;
    expand into a-z order via a Buffer

For 'eat', 'tea', 'ate' -> all canonicalise to 'aet'. For 'tan',
'nat' -> 'ant'. For 'bat' -> 'abt'.

group_anagrams folds the input, accumulating per-key string lists;
final answer is Hashtbl.length (number of distinct groups):

  ['eat'; 'tea'; 'tan'; 'ate'; 'nat'; 'bat']  -> 3 groups

99 baseline programs total.

plans/ocaml-on-sx.md

@@ -407,6 +407,14 @@ _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 5.1 — anagram_groups.ml baseline (group strings
+  by canonical anagram form, ["eat";"tea";"tan";"ate";"nat";"bat"]
+  has 3 groups). canonical builds a sorted-by-frequency string
+  representation: count letters, then expand into a-z order. Used
+  as Hashtbl key. group_anagrams folds the input list, accumulating
+  per-key string lists; final answer is Hashtbl.length (number of
+  distinct groups). Tests count-then-expand canonical pattern +
+  Hashtbl as multimap. 99 baseline programs total.
 - 2026-05-09 Phase 5.1 — monotonic.ml baseline (monotonicity check,
   4/5 inputs monotonic). Tracks two bool refs (inc, dec). Each pair
   of consecutive elements: if `h < prev` clear `inc`, if `h > prev`