ocaml: phase 6 Either module + Hashtbl.copy (+4 tests, 602 total)

Either module (mirrors OCaml 4.12+ stdlib):
  left x / right x
  is_left / is_right
  find_left / find_right (return Option)
  map_left / map_right (single-side mappers)
  fold lf rf e            (case dispatch)
  equal eq_l eq_r a b
  compare cmp_l cmp_r a b (Left < Right)

Constructors are bare 'Left x' / 'Right x' (OCaml 4.12+ exposes them
directly without an explicit type-decl).

Hashtbl.copy:
  build a fresh cell with _hashtbl_create
  walk _hashtbl_to_list and re-add each (k, v)
  mutating one copy doesn't touch the other
  (Hashtbl.length t + Hashtbl.length t2 = 3 after fork-and-add
   verifies that adds to t2 don't appear in t)

plans/ocaml-on-sx.md

@@ -407,6 +407,13 @@ _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 — Either module + Hashtbl.copy (+4 tests, 602
+  total). Either: left, right, is_left, is_right, find_left,
+  find_right, map_left, map_right, fold, equal, compare. Constructors
+  are bare `Left x` / `Right x` (per OCaml 4.12+). Hashtbl.copy
+  builds a fresh cell, walks `_hashtbl_to_list`, and re-adds; mutating
+  one copy doesn't touch the other (verified by `Hashtbl.length t +
+  Hashtbl.length t2 = 3` after a fork-and-add).
 - 2026-05-09 Phase 5.1 — json_pretty.ml baseline (recursive ADT
   serialization). Defines a JSON-like ADT (JNull / JBool / JInt /
   JStr / JList) and recursively pretty-prints to a string, then