ocaml: phase 6 Random module (deterministic LCG) (+4 tests, 549 total)

Linear-congruential PRNG with mutable seed (_state ref). API:

  init s        seed the PRNG
  self_init ()  default seed (1)
  int bound     0 <= n < bound
  bool ()       fair coin
  float bound   uniform in [0, bound)
  bits ()       30 bits

Stepping rule:
  state := (state * 1103515245 + 12345) mod 2147483647
  result := |state| mod bound

Same seed reproduces the same sequence. Real OCaml's Random uses
Lagged Fibonacci; ours is simpler but adequate for shuffles and
Monte Carlo demos in baseline programs.

  Random.init 42; Random.int 100  = 48
  Random.init 1;  Random.int 10   = 0

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 6 — Random module (LCG-based, deterministic) (+4
+  tests, 549 total). Linear-congruential PRNG with mutable seed
+  (`_state` ref). API: init, self_init, int, bool, float, bits.
+  `int bound` returns `|state| mod bound` after stepping. Same seed
+  reproduces same sequence — useful for testing shuffles and Monte
+  Carlo demos. Real OCaml's Random uses Lagged Fibonacci; ours is
+  simpler but adequate for baseline programs.
 - 2026-05-09 Phase 6 — Hashtbl.keys / values / bindings / remove /
   clear / reset / to_seq / to_seq_keys / to_seq_values (+4 tests, 545
   total). Two new host primitives `_hashtbl_remove` and