ocaml: phase 5.1 huffman.ml baseline (Huffman tree WPL = 224)

Classic CLRS Huffman code example. ADT:

  type tree = Leaf of int * char | Node of int * tree * tree

Build by repeatedly merging two lightest trees (sorted-list pq):

  let rec build_tree lst = match lst with
    | [t] -> t
    | a :: b :: rest ->
      let merged = Node (weight a + weight b, a, b) in
      build_tree (insert merged rest)

  weighted path length (= total Huffman bits):
    leaves {(5,a) (9,b) (12,c) (13,d) (16,e) (45,f)} -> 224

Tests sum-typed ADT with mixed arities, `function` keyword
pattern matching, recursive sorted insert, depth-counting recursion.

150 baseline programs total.

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.
+- 2026-05-10 Phase 5.1 — huffman.ml baseline (Huffman tree weighted
+  path length on letters {(5,a) (9,b) (12,c) (13,d) (16,e) (45,f)}
+  = 224). Builds optimal prefix code by repeatedly merging the two
+  lightest trees: insert merged node back into a sorted-by-weight
+  list. Verifies the standard Huffman result of 224 bits for this
+  classic CLRS example. Tests sum-typed ADT with two arities
+  (Leaf of int * char | Node of int * tree * tree), `function`
+  keyword pattern matching, recursive sorted insert, depth-counting
+  recursion. 150 baseline programs total.
 - 2026-05-10 Phase 5.1 — parser: accept `if/match/let/fun/...` as
   the rhs of `<-` and `:=`. parse-binop-rhs now special-cases prec-1
   ops (`<-`, `:=`) to call parse-expr-no-seq for their right operand