ocaml: phase 5.1 caesar.ml baseline (ROT13 + s.[i] + Char ops)

Side-quests required to land caesar.ml:

1. Top-level 'let r = expr in body' is now an expression decl, not a
   broken decl-let. ocaml-parse-program's dispatch now checks
   has-matching-in? at every top-level let; if matched, slices via
   skip-let-rhs-boundary (which already opens depth on a leading let
   with matching in) and ocaml-parse on the slice, wrapping as :expr.

2. runtime.sx: added String.make / String.init / String.map. Used by
   caesar.ml's encode = String.init n (fun i -> shift_char s.[i] k).

3. baseline run.sh per-program timeout 240->480s (system load on the
   shared host frequently exceeds 240s for large baselines).

caesar.ml exercises:
  * the new top-level let-in expression dispatch
  * s.[i] string indexing
  * Char.code / Char.chr round-trip math
  * String.init with a closure that captures k

Test value: Char.code r.[0] + Char.code r.[4] after ROT13(ROT13('hello')) = 104 + 111 = 215.

lib/ocaml/baseline/expected.json

@@ -2,6 +2,7 @@
   "anagrams.ml": 3,
   "bfs.ml": 6,
   "btree.ml": 39,
+  "caesar.ml": 215,
   "calc.ml": 13,
   "closures.ml": 315,
   "exception_handle.ml": 4,