ocaml: phase 5.1 xor_cipher.ml baseline (XOR roll-key encryption, round-trip = 601)

For each character, XOR with the corresponding key char (key cycled
via 'i mod kn'):

  let xor_cipher key text =
    let buf = Buffer.create n in
    for i = 0 to n - 1 do
      let c = Char.code text.[i] in
      let k = Char.code key.[i mod kn] in
      Buffer.add_string buf (String.make 1 (Char.chr (c lxor k)))
    done;
    Buffer.contents buf

XOR is its own inverse, so encrypt + decrypt with the same key yields
the original. Test combines:
  - String.length decoded            = 6
  - decoded = 'Hello!'                  -> 1
  - 6 * 100 + 1                          = 601

Tests Char.code + Char.chr round-trip, the iter-127 lxor operator,
Buffer.add_string + String.make 1, and key-cycling via mod.

90 baseline programs total.

plans/ocaml-on-sx.md

@@ -407,6 +407,13 @@ _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 — xor_cipher.ml baseline (XOR roll-key
+  encryption, round-trip → 601). For each character, XOR with the
+  corresponding key char (key cycled via `i mod kn`). Encrypts
+  "Hello!" with key "key", decrypts the result, and verifies the
+  round-trip preserves both length (6) and equality. Tests
+  Char.code + Char.chr round-trip + the iter-127 `lxor` operator
+  + Buffer.add_string + String.make 1. 90 baseline programs total.
 - 2026-05-09 Phase 5.1 — simpson_int.ml baseline (Simpson's rule
   numerical integration, ∫₀¹ x² dx ≈ 1/3, scaled = 10000). Composite
   Simpson's 1/3 rule with 100 panels. Coefficients 1-4-2-...-2-4-1