ocaml: phase 6 Char predicates (+7 tests, 461 total)

Char.is_digit / is_alpha / is_alnum / is_whitespace / is_upper /
is_lower / is_space — all written in OCaml using Char.code + ASCII
range checks.

plans/ocaml-on-sx.md

@@ -407,6 +407,9 @@ _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.
+- 2026-05-08 Phase 6 — Char predicates: is_digit/is_alpha/is_alnum/
+  is_whitespace/is_upper/is_lower (+7 tests, 461 total). All written
+  in OCaml in runtime.sx using Char.code + ASCII range checks.
 - 2026-05-08 Phase 5 — HM for top-level `let..and..` decls (+3
   tests, 454 total). `ocaml-type-of-program` now handles `:def-mut`
   (sequential generalization) and `:def-rec-mut` (mutual recursion