ocaml: phase 5.1 bag.ml baseline + String.equal/compare/cat/empty (+3 tests, 579 total)

bag.ml: split a sentence on spaces, count each word in a Hashtbl,
return the maximum count via Hashtbl.fold.

  count_words 'the quick brown fox jumps over the lazy dog the fox'
  -> Hashtbl with 'the' = 3 as the max
  -> 3

Exercises String.split_on_char + Hashtbl.find_opt/replace +
Hashtbl.fold (k v acc -> ...). Together with frequency.ml from
iter 84 we now have two Hashtbl-counting baselines exercising
slightly different idioms. 29 baseline programs total.

String additions:
  equal a b      = a = b
  compare a b    = -1 / 0 / 1 via host < / >
  cat a b        = a ^ b
  empty          = '' (constant)

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 — bag.ml baseline + String.equal/compare/cat/
+  empty (+3 tests, 579 total). bag.ml: split a sentence on spaces,
+  count word frequency in a Hashtbl, return the maximum count.
+  Sentence "the quick brown fox jumps over the lazy dog the fox" has
+  "the"×3 as the most frequent → 3. Exercises String.split_on_char +
+  Hashtbl.find_opt/replace + Hashtbl.fold over (k, v) tuples. 29
+  baseline programs total. String additions: equal, compare (via host
+  `<`/`>`), cat (alias of `^`), empty.
 - 2026-05-09 Phase 5.1 — fraction.ml baseline (rational arithmetic
   via record + gcd canonicalization). Defines `type frac = { num;
   den }`, `make` that reduces via gcd and forces den > 0, `add` and