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.

lib/ocaml/baseline/expected.json

@@ -1,4 +1,5 @@
 {
+  "calc.ml": 13,
   "closures.ml": 315,
   "exception_handle.ml": 4,
   "expr_eval.ml": 16,