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.

plans/ocaml-on-sx.md

@@ -407,6 +407,17 @@ _Newest first._
   binary search tree (`type 'a tree = Leaf | Node of 'a * 'a tree *
   'a tree`) with insert + in-order traversal. Tests parametric ADT,
   recursive match, List.append, List.fold_left.
+- 2026-05-09 Phase 5.1 — caesar.ml baseline (ROT13 with String.init +
+  s.[i] + Char.code/chr). Side-quests:
+  (1) top-level `let r = expr in body` is now treated as an expression
+  decl when has-matching-in? returns true at the dispatcher. Slices via
+  skip-let-rhs-boundary which already opens depth on a leading let
+  with matching in;
+  (2) added String.make / String.init / String.map to runtime;
+  (3) bumped lib/ocaml/baseline/run.sh per-program timeout 240→480s
+  for headroom on contended hosts.
+  Test = `Char.code r.[0] + Char.code r.[4]` after ROT13 round-trip on
+  "hello" → 215 (h+o).
 - 2026-05-08 Phase 4 — `s.[i]` string indexing syntax (+3 tests, 484
   total). parse-atom-postfix now handles `.[expr]` after `.`,
   emitting `(:string-get S I)`; eval reduces to host `(nth s i)`.