ocaml: phase 5 HM let-mut / let-rec-mut (+3 tests, 442 total)

ocaml-infer-let-mut: each rhs inferred in parent env, generalized
sequentially before adding to body env.

ocaml-infer-let-rec-mut: pre-bind all names with fresh tvs; infer
each rhs against the joint env, unify each with its tv, then
generalize all and infer body.

Mutual recursion now type-checks:
  let rec even n = if n = 0 then true else odd (n - 1)
  and odd n = if n = 0 then false else even (n - 1)
  in even   : Int -> Bool

plans/ocaml-on-sx.md

@@ -399,6 +399,12 @@ _Newest first._
   recognise `!` as the prefix-deref of an application argument, so
   `String.concat "" (List.rev !b)` parses as `(... (deref b))`. Buffer
   uses a ref holding a string list; contents reverses and concats.
+- 2026-05-08 Phase 5 — HM let-mut / let-rec-mut inference (+3 tests,
+  442 total). `ocaml-infer-let-mut` infers each rhs in the parent env
+  and generalizes sequentially; `ocaml-infer-let-rec-mut` pre-binds
+  all names with fresh tvs, infers each rhs against the joint env,
+  unifies, generalizes, then infers body. Mutual recursion works:
+  `let rec even n = ... and odd n = ... in even : Int -> Bool`.
 - 2026-05-08 Phase 6 — Option/Result/Bytes extensions (+9 tests, 439
   total). Option: join, to_result, some, none. Result: value, iter,
   fold. Bytes: length, get, of_string, to_string, concat, sub (thin