ocaml: phase 5.1 calc.ml baseline (11/11 pass) + inline let-rec-and parser fix

Recursive-descent calculator parses '(1 + 2) * 3 + 4' = 13. Two parser
bugs fixed:

1. parse-let now handles inline 'let rec a () = ... and b () = ... in
   body' via new (:let-rec-mut BINDINGS BODY) and (:let-mut BINDINGS
   BODY) AST shapes; eval handles both.

2. has-matching-in? lookahead no longer stops at 'and' — 'and' is
   internal to let-rec, not a decl boundary. Without this fix, the
   inner 'let rec a () = ... and b () = ...' inside a let-decl rhs
   would have been treated as the start of a new top-level decl.

Baseline exercises mutually-recursive functions, while-loops, ref-cell
imperative parsing, and ADT-based AST construction.

plans/ocaml-on-sx.md

@@ -394,6 +394,14 @@ _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 5.1+1+2 — calc.ml baseline (11/11 pass) — a
+  recursive-descent calculator parsing `(1 + 2) * 3 + 4` to 13. Two
+  parser bugs fixed along the way: parse-let now handles inline
+  `let rec ... and ... in body` via new `:let-rec-mut` / `:let-mut`
+  AST shapes (eval supports both); `has-matching-in?` no longer stops
+  at `and` (which is internal to a let-rec, not a decl boundary). The
+  baseline exercises mutually-recursive functions, while-loops, and
+  ref-cell-driven imperative parsing.
 - 2026-05-08 Phase 5.1 — word_count.ml baseline (10/10 pass). Uses
   Map.Make(StrOrd) + List.fold_left to count word frequencies; tests
   the full functor pipeline with a real OCaml idiom.