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/runtime.sx

@@ -360,6 +360,20 @@
      let split_on_char c s = _string_split_on_char c s
      let replace_all s a b = _string_replace s a b
      let index_of s sub = _string_index_of s sub
+     let make n c =
+       let rec aux i acc = if i = 0 then acc else aux (i - 1) (acc ^ c) in
+       aux n \"\"
+     let init n f =
+       let rec aux i acc =
+         if i >= n then acc else aux (i + 1) (acc ^ f i)
+       in
+       aux 0 \"\"
+     let map f s =
+       let rec aux i acc =
+         if i >= _string_length s then acc
+         else aux (i + 1) (acc ^ f (_string_get s i))
+       in
+       aux 0 \"\"
    end ;;
 
    module Bytes = struct