ocaml: character type — Char of int, #\a parser, all char primitives

- Char of int variant in sx_types.ml (Unicode codepoint)
- type_of → "char", inspect → #\a / #\space / #\newline notation
- #\ char literal reader in sx_parser.ml (named + single-char)
- make-char char? char->integer integer->char char-upcase char-downcase
- char=? char<? char>? char<=? char>=? comparators
- char-ci=? char-ci<? char-ci>? char-ci<=? char-ci>=? case-insensitive
- char-alphabetic? char-numeric? char-whitespace? char-upper-case? char-lower-case?
- string->list (returns Char values) and list->string (accepts Char values)
- fix get_val in sx_runtime.ml: add Integer n case for list indexing
- fix raw_serialize in sx_server.ml: Integer and Char variants
- 4493/4493 tests — +43 passing, zero regressions

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

hosts/ocaml/lib/sx_runtime.ml

@@ -211,6 +211,8 @@ let get_val container key =
   | Dict d, Keyword k -> dict_get d k
   | (List l | ListRef { contents = l }), Number n ->
     (try List.nth l (int_of_float n) with _ -> Nil)
+  | (List l | ListRef { contents = l }), Integer n ->
+    (try List.nth l n with _ -> Nil)
   | Nil, _ -> Nil  (* nil.anything → nil *)
   | _, _ -> Nil    (* type mismatch → nil (matches JS/Python behavior) *)